There is a key word: recharge.

We require energy to do countless activities (intellectual and physical). It implies a physiological wear which could be fatal if accumulates. That is why our body needs to get the time to repair itself, and that time comes with sleep.

Great question!

Most bacteria lives in environments which from the viewpoint of the bacteria interaction do not vary on a daily basis and so bacteria do not sleep. Most higher animals cannot function in the dark, at night; their vision is impaired and this is probably one of the reasons why the daily sleep cycle evolved and certain functions later evolve to take advantage of this period of rest.

Sleep is not just a "passive" process. We have to understand also as an active state, that implies more than resting, relaxing, restoring, inhibition or standing by. SNC reprogramming, memory consolidation or neurochemical "cleansing" are just a few examples. Anyway, have you ever thought that maybe is wakefulness that claims an explanation (from an ancestral evolutionary point of view)?

Could sleep be the default state for the very ancient living beings?

There is a key word: recharge.

We require energy to do countless activities (intellectual and physical). It implies a physiological wear which could be fatal if accumulates. That is why our body needs to get the time to repair itself, and that time comes with sleep.

Great question!

Sleep is necessary for your brain and body, its like charging your mobile to use the entire day, similarly sleep charges you for an entire day.

Dear Ziad

Sleep is characterized by " slow down metabolism , muscle relaxation and decreased sensory activity . " However , despite seeming a time when just relaxing , sleep is responsible for many physiological functions of the body .

It is during sleep also , that there is significant production of the hormone GH , responsible for the growth of children. And this hormone , in turn , serves to activate another key hormone responsible for " protein synthesis in bones and muscles and absorption of nutrients," IGF - 1 . This pleasant night time is still responsible for the maturation of the nervous system , thus making sleep quality , a key ally in the development of children and adolescents .

But it is not only children who enjoy advantages when sleeping . For adults , sleep has a restorative potential energies and acts for the proper development of memory . The lack of it can lead to problems , depression , irritability and fatigue.

Therefore , some measures that ensure quality sleep should be taken as a light diet , adequate lighting . Other attitudes must be avoided , such as stimulating beverages and physical exercise before sleep

Http://news.sciencemag.org/brain-behavior/2013/10/sleep-ultimate-brainwasher

Apparently it is related to the removal of neurotoxic waste from daytime mental activity. See details here: http://www.sciencemag.org/content/342/6156/373

Ziad et al., sleep is as much needed as work and activity as Nelson, Richard have discussed. Our cells and its DNA is repaired, harmful wastes are converted into products that can be removed from our bodies.

To ensure that I sleep well, I work hard throughout the day and exercise in the evenings, but not too close to bedtime. I don't have a nap at all, even during weekends, since I started teaching. Some years ago, our time-zone for West Malaysia was brought forward by a half-hour, so that the whole country follows the same time zone. That means an extra half hour of daylight for jogging and walking.

To unwind, I read and pray. Recently, I have been reading RG posts before sleep. It's the time when our Europe and US friends are busy putting up posts. It's a great way to unwind :)

Dear Dr. Abu-Faraj,

I have watched a very interesting talk about "Why do we sleep?". It is a TEDx talk that you can find on the following links:

http://www.ted.com/talks/russell_foster_why_do_we_sleep.html

http://www.youtube.com/watch?v=LWULB9Aoopc

I hope this helps in answering your question.

Dear All:

I was so thrilled to receive this magnitude of interesting answers in a very short time. Attention should be paid to Nelsson's and Miranda's answers in that during sleep several functionalities are occuring besides the normal resting and recharging. This fact makes sense in terms of efficiency; i.e., the one-third part of the day is not only wasted on revitalizing but on other useful functions that do not happen during the day. Also, adaptation to the day/night cycle is an interesting concept.

Thanks Heidi - Nice article!

Thanks Michel for the interesting links!

A wonderful question and for very sure a challenging research field :)

Let me please give me a private hopefully funny answer:

For me sleep is nice if I remember a realistic FUN-tastic dream, AND, sometimes

I remember suddenly a research problem, make a note (awake) and see in the LAB... it works! :) (sometimes of course not).

Good discussion...

Well, I would say simply - recharging those batteries!?

Regards

Theodora Issa

Dear Louis Brassard:

Your argument about the daily sleep cycle in higher animals to possibly have evolved to take advantage of the night period as a period of rest is quite interesting.

In your educated opinion what is the effect on the sleep/wake cycles for people residing in Scandinavian countries or polar regions whereby the light/dark cycle spans for several months during the year?

Dear Ziad,

I did not do an extensive search in order to know what is the establish scientific knowledge supporting this hypothesis but the fact that most animals that sleep , do it during the nightly period is sudgestive. But after hundred of million of years of evolution, biological system certainly have evolved all kind of functions taking advantage of this period of sleep, making sleep necessary for the performance of a large variety of biological functions that are programmed to be taking place during sleep. Birds and Mammals for example have evolved dreaming, or REM sleep in order to optimize learning from experience, the growth of long term memory. During dreaming, the sensory-motor system is put into an offline mode where some kind of simulations are taking place in order to consolidated and optimize learning from the experience of the day.

In the canadian artic, there is no daily light cycles and there in the inuit villages people sleep like elsewhere but they do not it at the same time. In the same house, people do not sleep at the same time. In the school, some children sleep on the benches. In there, there is no compelling insentive for every body to sleep at night since there is no period of the day corresponding to it.

I am Scandanavian and every member of my extended family that I can recall "suffered" from seasonal affective disorder. We tend to become moody when the days are shortened or when there is little light. Interesting thinking...

What is being taught in our medical curriculumn at the moment, is that sleep plays a vital role in the learning process. Neurotransmitters, such as acetylcholine and norepinephrine shift in concentrations from day to night. These up and down regulations contribute to rerouting neural networks as to solidify memory, or rather, new inputs learned throughout the day. This procees takes an enormous amount of enrgy (ATP), so we cannot be physically active and store vast amounts of stimuli at the same time. Also, sleep is finely tuned to endocrine cycles. Therefore, sleep is heavily involved in the neuroendocrine regulation and actvity.

Sleep research is interesting domain to work, i am not a neuroscientist although. Definitely, sleep recharges the body and mind, and that appears to be the primary function of sleep process. However, sleep also take us (our mental attention) away from physical world, into dreams, which i believe do have some purpose when our subconscious brain takes over.

Also, it has been proposed by studies that sleep helps brain to do a downscaling of memories at the level of synapse, so that unimportant memories are deleted to conserve space for new memories

(Vyazovskiy VV, Cirelli C, Pfister-Genskow M, Faraguna U, Tononi G. Nat Neurosci. 2008 Feb;11(2):200-8).

As other biological processes and organs have not just one function, sleep must be having other secondary functions too and needs to be studied further.

Carla,

Very interesting comments - is there anything that is done about that...

Regards

Theodora Issa

Dears Louis, Carla, Michael, Tarun, et al.

Thank you for the information that added value to the discussion pertaining to this question.

Thank you Anam and Theodora!

I believe that (at least by observing my own habits) sleep is a "self-healing" process which involves physiology and psychology. There is a reordering of the memory, not necessarily forgetting rather moving things from top priority to lowest priority. I am able to program myself to sleep and wake up per hour or set of hours and feel great after each batch! Very important you need to establish contact with one's senses and understand the body signals. My body tells me go to sleep (even for 2 to 3 hours) and when up it is strong and running.

So, it may have a scientific explanation, but I feel I have to sense what my body wants specially sleeping. I also use sleeping for recharging the energy to solve problems (even complicated engineering ones!). It is amazing that many times I sleep with many alternative solutions in mind and I wake up with the appropriate.

So, even with my sleep habits I have fun.

(:-)))

Purpose of sleep according to me :

1. Sleeping is needed for transferring of data in temporary memory to the permanent memory i.e. subconscious mind so that it can be recalled in future when necessary.

2. Also you can get solution of your many of your real life problems in your dreams during sleeping.

3. It gives relaxation to your mind and body.

4. It is also necessary for different metabolism of our body.

Regards,

Nitish

Ziad one of the more complex questions on the planet I would think! I deal with sleep and its resultant effect on performance so I can offer only my viewpoints from a physiological perspective and a rather hashed attempt from a neural perspective. Anywho, sleep is a re-occurring behavioural state believed to be critical to human physiological function and performance. In spite of this perceived importance, the consensus specifically with regards to the functionality of why humans sleep remains fairly equivocal. Several theoretical models of the function of sleep have been presented, including the restoration, energy conservation and memory consolidation models of sleep. Whilst a complex interaction between these models is highly probable when humans are forced to cope with physiological and psychological stress, the restorative model has promoted the strongest evidence for sleep to play a role in physiological recovery. Personally, although I work with physiological recovery and thus the restoration model somewhat, the neural model seems to embody the strongest evidence. Regardless, these models, similar to the many complex models proposed about fatigue, work in conjunction with one another to produce (or attempt to produce) homeostasis. Indeed, when humans sleep, growth hormone is released, stimulating protein synthesis. This process can provide a restorative effect to repair peripheral muscular damage. Sleep can also assist several thermoregulatory processes to provide energy conservation and muscular restoration for the body and has even been shown to accelerate the rate of healing following tissue damage. Recently, it has been confirmed sleep it critical to metabolic homeostasis. Since these critical metabolic and immune processes occur during sleep, a relationship appears to exist between the amount, quality and depth of sleep and the capacity of individuals whom exercise to perform and recover, however evidence of this in elite athletic populations is almost non-existent.

Thanks Hugh for your very valuable insight!

Some aspects of sleep which have not been specifically discussed here are:

1- Sleep disorders.

2- Mental disorders associated with sleep such as schizophrenia.

3- The power of healing that sleep has - from headaches to serious illnesses.

I would like to invite everyone to this discussion.

Every aspect of sleep appears to be important and needs to be explored. We have rightly mentioned many aspects like functions of sleeps, physiological (including endocrine and metabolic) and psychological requirements fullfilled by sleep.

However, one significant point which i have observed personally is TIMING. I am sure many of us would also agree that there is a 'Right Time Window' of sleeping, and if u miss that window, then you may be more tired later, but when u sleep after that window you wont get to sleep easily and even if u do, u dont get that level of energy after wake up which u do by sleeping in 'Right Time Window' (RTW) sleep.

I believe the graph between "Energy after wake up" (at y axis) and duration of tiredness/ or time before sleeping is Bell shaped curve, which means there must be a RTW at which the Energy gained due to sleep is maximal (curve peak).

In short , the Timing of sleep is very important.

Dear Ziad,

Great topic to discuss, thanks.

Sleep has been also associated to increased creativity and problem solving via studies on relational memory

"Ellenbogen et al. 2007, PNAS, Human relational memory requires time and sleep".

http://www.ncbi.nlm.nih.gov/pubmed/?term=ellenbogen+sleep+2007

PDF: http://walkerlab.berkeley.edu/reprints/Ellenbogen&Walker_PNAS_2007.pdf

On a lighter side sleep and dreams have been at the origin of various discoveries ranging from the Benzene ring structure by August Kekulé or the functioning of synapses by Otto Loewi and even Dmitri Mendeleev's periodic table to song writing (for the Beatles fans, the tune for "Yesterday" came to Paul McCartney in a dream ;) )

http://www.nature.com/nature/journal/v408/n6812/full/408523a0.html

Sleep disorders are many and can seriously alter performance, physiology and well being. Sleep apnea, as an example, is linked to fatigue and sleepiness during the day as well as possible heart problems. Insomnia, however, remains the most common sleep problem and is observed in patients with schizophrenia. In fact, difficulties initiating or maintaining sleep are frequently encountered in these patients as well as alteration in sleep architecture. Sleep problems have also been suggested to precede Parkinson's disease.

As mentioned previously sleep deprivation leads to abnormal bone growth and cleansing of toxic molecules in the brain (such as Lewy bodies in Alzheimer's). Interestingly patient's with Alzheimer's sleep poorly. Could that be leading to the increase in Lewy bodies and therefore possibly worsening amnesia?

We don't know however, if these plaques are causing the pathology or are just symptoms.

Regarding sleep in Scandinavian and polar regions Carla's comment is very interesting. Seasonal affective disorders are also common on other latitudes of the globe. They are due to a decrease or sometimes increase (less frequent) in the amount of light received due to the shortening or lengthening of the light episode. Moreover, alterations in sleep and circadian patterns have been recorded in polar stations either in the Arctic or Antarctic. All this is probably due to the absence of sunlight and aberrant indoor lighting exposures.

We recently finished a study in the Concordia Antarctic polar station in which we tested the impact of specially designed indoor lights on sleep, circadian rhythms and neurobehavioral functions. This study was done during polar winter when you basically have 3 month of darkness due to the lack of environmental sunlight. The paper is currently under revision and I would be happy to share it with you soon.

Best,

Very enlightening contribution Raymond. I want to bring forward another possible factor that may affect sleep. How about electromagnetic fields? Some residents living under or near power stations or high voltage electrical cables claim that they suffer from sleeping disorders!

Dear Raymond:

Thank you for your insightful contribution to this question and for elaborating on the effect of sleep disorders on sleep.

I look forward to reading your paper on sleep during polar winter.

Regards,

ZAF

The Effects of Electromagnetic Fields on Sleep - Good point Hussin!

I'm not even close to the field, but I think a short answer would be 'maintenance'.

I guess we can extend the discussion to "why sleep at night" or "why does the body need total darkness during sleep", and this can lead us to the hormonal processes that take place in the body during sleep that requires 'darkness'.

Yes indeed Murat!

One of the important functions of sleep is for the consolidation of memory as mentioned by Hugh Fallagar. Correlates well with Raymond Najjar's comment that patient's with Alzheimer's disease sleep poorly.

Thanks Anand!

@Ziad thanks again for the discussion, will send you the paper as soon as it gets accepted. Fingers crossed.

@Hussin, thanks for your comment! yes the impact of electromagnetic fields (EMF) is still highly debated but its seems that these fields alter melatonin secretion which in return could negatively impact sleep. That might be something to study in Lebanon?

http://www.ncbi.nlm.nih.gov/pubmed/23051584

EMFs also seem to one degree or another increase the risk of childhood leukemia, adult brain cancer, Lou Gehrig's disease and miscarriage but that's another topic.

@Anam, thanks for your comment ! We do dream every night but we only recall some or sometimes none of our dreams. We mainly dream in REM (rapid eye movement) sleep but we might also dream in non-REM Slow wave sleep. The chances of remembering a dream are usually higher if you awake during a REM episode and the question is which dreams do we recall. Good ones or bad ones. Now is healthy sleep in relation with pleasant dreaming and not nightmares? I would think so. Patients with insomnia have a tendency to recall bad dreams. It is also a vicious circle. If you have a sleep disorder like night terrors, insomnia or just common nightmares then you are prone to wake up more often and loose sleep and alter your sleep architecture. This would probably lead to a REM rebound on the second day and more dreams... bad ones or good ones? which do we recall?. Traumatic experiences or stress do alter sleep and could affect dreams in fact people who suffer from post-traumatic stress disorder (PTSD) experience painful nightmares.

@ Hi Murat, we sleep at night because we are synchronized to the earth's and artificial light dark cycle. Light in the morning makes you sleep earlier on the following night whereas light prior to sleep makes you sleep later. This is dependent of course upon the intensity and color content of light. The balance is essential for what we call entrained 24h hour rhythm (like the earth's 24h light dark cycle rhythm). Dark is advised for sleep as light suppresses the pineal gland hormone melatonin and increases alertness (probably via both melatonin suppression and projections from the retina to the arousal centers of the brain).

Some people, however, do not sleep at night. As an example: blind individuals who are not entrained/synchronized by the light dark cycle or even shift workers who willingly shift their wake/rest schedule.

Best

Ziad et al.,

I feel I must share with you what I learn from the Bible about sleep. Tell me what you think, ok?

It says 'the sleep of the laborer is sweet'. My research is a labor of love and a challenge, because unlike many of you, I don't get a grant. I have stopped complaining and just press on. The good thing is that I don't get pressure, as long as I investigate 'learning'. NO one says to me, 'Publish or perish'. But on RG, so far, I'm the only one from my institution.

So coming back to the topic, laborers like me sleep to get the sweetness.

I think it is a Hibernation Process for the Human brain

Miranda,

What a great inclusion from the Bible... (Proverbs)... indeed... well, SLEEP in the Bible would also refer to REST as the Christians believe that those who depart this life go to SLEEP - and REST - that is if we take the word SLEEP in the bible to its original Aramaic ....

From the New Testament John 11:12 - Then said his disciples, Lord, if he sleep, he shall do well.

Roger, Hibernation - a technological expression... good one!

Regards

Theodora Issa

Http://www.sciencemag.org/content/342/6156/373

in my the purpose of sleep is, it is the need of body which naturally done at night but nowadays the time is being change every person have there own but mostly people for relaxation.

it is a God gifted need based process which mean to relaxation but we change it only to relaxation not doing for the need of body.....

Dear Ziad and colleagues,

As promised a while ago

"We recently finished a study in the Concordia Antarctic polar station in which we tested the impact of specially designed indoor lights on sleep, circadian rhythms and neurobehavioral functions. This study was done during polar winter when you basically have 3 month of darkness due to the lack of environmental sunlight. The paper is currently under revision and I would be happy to share it with you soon."

Please find a link to our recently published study. Feel free to get back to me with any comments.

Best wishes.

-Ray

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