1. organ cleansing: Our brain is the organ with the largest energy consumption per weight. The resulting metabolic deposits accumulate over the day within the tissue. Recent research (http://news.sciencemag.org/brain-behavior/2013/10/sleep-ultimate-brainwasher) reveal that there are tiny channels which are perfused during sleep and "wash" our brain.

2. memory reorganisation: After about 1,5h of non-REM sleep (cleansing) our brain starts to be especially active (REM sleep). During this phase, memory and yesterday's experiences are reorganized with a freshly washed brain.

1 and 2 are repeated until the morning 3-4 times, until the brain is fully reorganized.

Therefore, an angry email you write in an exhausted state before going to bed reads quite different in the morning - never send such an email before having slept over it ;-)

Autonomic nervous system nicely shows the alternation of phases of parasympathetic (cleansing non-REM sleep) and sympathetic activity (reorganizing REM sleep), see Fig 4 or 7 in this paper: https://www.researchgate.net/publication/3246475_The_Symphony_of_Life_Chronobiological_Investigations?ev=prf_pub

Kind regards

Max

Article The symphony of life - Importance, interaction, and visualiz...

Just a slight clarification on the first part of your question. It is not only humans and advanced species that need sleep. Also less advanced species like fruit flies seem to need sleep. In fact, it appears to be having a brain or not that is the key factor. Once a species has a brain (even when it is only a few neurons in size), it needs sleep. For the role of sleep in learning and memory, I refer you to the work of, for instance, Jan Born. A good starting point would, for instance, be "Diekelmann S, Born J (2010). The memory function of sleep. Nature Rev Neurosci. 11(2):114-26."

My theory is that at first, sleep was an adaptive mechanism to reduce motor activity, reduce metabolic demands and a escape from predators in quiet hidden places. After times, evolution associated the advantage of compensatory mechanisms to consolidate memory and refresh neural circuitry. Other functions such as hormonal secretion and etc occurred. The puzzle to solve is why temperature oscillates in the 24 hour cicle. What temperature variation does to the brain? The brain matters!!! What is the point in good cardiovascular functioning and aging without the brain.

1. organ cleansing: Our brain is the organ with the largest energy consumption per weight. The resulting metabolic deposits accumulate over the day within the tissue. Recent research (http://news.sciencemag.org/brain-behavior/2013/10/sleep-ultimate-brainwasher) reveal that there are tiny channels which are perfused during sleep and "wash" our brain.

2. memory reorganisation: After about 1,5h of non-REM sleep (cleansing) our brain starts to be especially active (REM sleep). During this phase, memory and yesterday's experiences are reorganized with a freshly washed brain.

1 and 2 are repeated until the morning 3-4 times, until the brain is fully reorganized.

Therefore, an angry email you write in an exhausted state before going to bed reads quite different in the morning - never send such an email before having slept over it ;-)

Autonomic nervous system nicely shows the alternation of phases of parasympathetic (cleansing non-REM sleep) and sympathetic activity (reorganizing REM sleep), see Fig 4 or 7 in this paper: https://www.researchgate.net/publication/3246475_The_Symphony_of_Life_Chronobiological_Investigations?ev=prf_pub

Kind regards

Max

Article The symphony of life - Importance, interaction, and visualiz...

This question has two main aspects. The first aspect concerns physiology. Specifically, during REM, the nervous system tunes some neural circuits in the brainstem regarding thermoregulation, hormonal balance etc. In other words, the nervous system is “updating its software”. The second aspect concerns cognition and it probably occurs only in humans. Humans are the only secondary-conscious animals. In this context, the work of Allan Hobson propose that the REM sleep is a protoconscious state, providing a virtual reality model of the world that is of functional use to the development and maintenance of waking consciousness (fro review see Hobson JA 2009, Nature Reviews Neuroscience 10, 803-813.)

http://www.nature.com/nrn/journal/v10/n11/abs/nrn2716.html

I would add to the other good comments that sleep is unlikely to be *just* for learning and memory, because animals (this has been carefully done for flies and rats) deprived of sleep die. It is difficult to understand how just impaired learning would lead to the complex metabolic/physiological distress and death of total sleep deprivation.

In the early '50s Dr. Winfried Otto Schumann discovered a resonance effect in the Earth - ionosphere system , now known by the term " Schumann Resonance " .

In physics this is called " cross - magnetic wave ."

It has been found that these waves vibrate in the same resonant frequency as the waves cereblales of humans ( and all mammals) , or 7.8 Hertz (cycles per second ) .

Generally, official physics used to ignore what you can not explain , but this time not followed this policy.

Germany and Austria have experienced with these resonances , have been developed and formulated new projects in physics Quantum 7.8 Hertz (frequency Schumann ) , considered these a constant biological Normal our hypothalamus.

A frequency that connects us to all human beings , where the Earth behaves like an enormous electric circuit.

This opens a "field of neural communication between two systems ." Hence , it would be considered the earth's electromagnetic field influence on sleep cycles .

There are two big theories on sleep and memory.

1. Sleep serves a purpose of memory consolidation - check papers from Ian Born (e.g. Boosting slow oscillations during sleep potentiates memory. Marshall L, Helgadóttir H, Mölle M, Born J. Nature. 2006 Nov 30;444(7119):610-3.) . According to this theory, hippocampal sharp waves and frontal ripples co-occur, representing temporal windows for consolidating hippocampal memories in the cortex. During these windows, sequences of awake hippocampal activity are replayed in a temporally compressed manner. See papers from the labs of Matthew Wilson and Bruce McNaughton, e.g. Biasing the content of hippocampal replay during sleep. Bendor D, Wilson MA. Nat Neurosci. 2012 Oct;15(10):1439-44. You can find useful reviews from Walker and Stickgold. (For instance, Sleep, memory, and plasticity. Walker MP, Stickgold R. Annu Rev Psychol. 2006;57:139-66.)

2. During sleep, synapses undergo downscaling to enable future memories. See papers from Giulio Tononi (Molecular and electrophysiological evidence for net synaptic potentiation in wake and depression in sleep. Vyazovskiy VV, Cirelli C, Pfister-Genskow M, Faraguna U, Tononi G. Nat Neurosci. 2008 Feb;11(2):200-8.). While the two theories seem contradictory at first, it is conceivable that some memories get consolidated and thus strengthened (with stronger synapses), but more synapses undergo 'maintenance' and reset to smaller synaptic weight, resulting in a net downscaling. This would enable synaptic homeostasis while retaining the important memories.

But it's important to realize that sleep is not only about the brain, and sleep deprivation leads to death in rodents through metabolic failure: Sleep and metabolism: shared circuits, new connections. Adamantidis A, de Lecea L. Trends Endocrinol Metab. 2008 Dec;19(10):362-70.

An evolutionary theory of sleep is worth noting: sleep evolved to conserve energy in the dark, when there's not much to do, and later a lot of functions (including brain and metabolic functions) were organized according to this cycle. (I don't know how this theory deals with different sleep patterns.)

In designing a first course in Sleep Epidemiology, I had included the following list at the beginning of Lecture 1. Does this list look correct? What would you add in a very basic list?

Antibodies are produced mostly during sleep. That’s why you need to sleep when you are sick.

Reparation substances are produced and the body is fixed. eg - when your eyes are changed over from dry and irritated to alert and feeling good after a good night’s sleep – that’s because new rejuvenating substances that were needed were produced: your eyes were actively fixed. And that’s why you need to sleep when you are injured.

Many hormones are produced mostly during sleep, e.g. ghrelin and leptin, which control appetite and metabolism, as well as growth hormone.

Memories and skills are organized and consolidated during sleep. And thinking never ceases: “waking up knowing something,” is NOT a myth

A recent article in Science highlights the role of sleep in sweeping the brain of toxic products accumulated in the nervous system when the organism is awake. In a study with mice, they show that natural sleep is associated with a 60% increase in the interstitial space, resulting in a striking increase in convective exchange of cerebrospinal fluid with interstitial fluid. See L. Xie, H. Kang et al., Sleep Drives Metabolite Clearance from the Adult Brain, Science, 18 October 2013: 342 (6156), pp. 373-377. [DOI:10.1126/science.1241224]

I would also suggest a classical paper by Ellenbogen et al on the role of sleep for memory, describing the evolution of the idea of sleep as simply protecting memories from interference to that of its actively contributing to consolidation.

http://www.ncbi.nlm.nih.gov/pubmed/17085038

The main function of sleep is to restore the brains metabolism.. Not to enhance learning or storage of information in LTM. To my knowledge there have been no controlled studies to date that have found evidence of a specific role of sleep in memory consolidation. Of course, there could be nonspecific effects like 'protection of interference', that explain why recall of material learned prior to sleep onset is superior after normal sleep than after sleep deprivation.

Good sleep and proper food are necessary for the well being of a human being, for building up immunity and for maintaining the body in good condition. Memory and learning are possible only if one is in good physical and mental condition.

But the question remains, why are memories not consolidated, synapses not normalised, or waste not removed while one is awake.

My guess is that sleep became first during evolution, perhaps because of it offered a benefit in saving energy during night when hunting or other activities would not have been as effective as during the day. Or, as Carl Sagan put it in his great book, The Dragons of Eden (1977): "The sleeping style of each organism is exquisitelyadapted to the ecology of the animal. It is conceivable that animals who are too stupid to be quiet on their own initiative are, during periods of high risk, immobilized by the implacable arm of sleep. The point seems particularly clear for the young of predatory animals; not only are baby tigers covered with asuperbly effective protective coloration, they also sleep a great deal."

After sleep cycles had been established, the organisms optimized other functions such as memory consolidation, synapse normalisation or waste-product transportation to the new rhythm of life. Any benefits from these may have further guided evolution so that the original benefit of sleep is now "forgotten", i.e., a new optimum has been found.

Thus, a feature or process (such as sleep) may have evolved for one purpose, but it presently serves a new purpose. We are prisoners of the old purpose.

Just a follow up of my prior comment. A dramatic demonstration of the restorative function of sleep is sleep deprivation. There have been some interesting studies on the effect of successive nights with little or no sleep at all of army personnel in the field. The no sleep soldiers suffered most in a task like rifle shooting. They had an estimated 4 days survival time in the field. But a few hours of sleep were sufficient to restore functions like the ability to concentrate, vigilance, short term memory, mood state etc.

It seems hard to imagine how to design a study that causes sleep deprivation without that experimental manipulation being confounded by the physiological facets of stress. Upon the relative merits of sleeping on the job and taking extra sleep: http://newscenter.berkeley.edu/2010/02/22/naps_boost_learning_capacity/ A hypothesis is that reductions in sleep we experience later in life contributes to cognitive ageing: http://www.ncbi.nlm.nih.gov/pubmed/23901074 Envision an elder looking out across the Masai-Mara; sentinel over sleeping children who the next day will be fresh for learning new skills to survive until one day they too can take the night watch.

A lesser known but dramatic effect of sleep is an ATP increase of brain areas that are associated with wakefulness, as if sleep was needed to restore the energetic charge of the brain. See J Neurosci. 2010 Jun 30;30(26):9007-16. doi: 10.1523/JNEUROSCI.1423-10.2010. Sleep and brain energy levels: ATP changes during sleep.

Dworak M, McCarley RW, Kim T, Kalinchuk AV, Basheer R.

It is true that the two dominant theories about sleep function are: 1)sleep consolidates experience, perhaps through 'replay' of waking experience and 2) sleep 'downscales' synapses during sleep to offset synaptic potentiation during wakefulness. However, there are two related caveats that should be kept in mind. First, 'replay' does not require sleep and can occur whenever an animal (or human, based on fMRI) is quietly awake. Therefore, it is not clear that this hypothesized mechanism for consolidation is really sleep-dependent. This may explain why some forms of memory do not need sleep at all for their consolidation. Second, while a lot of evidence has been collected in support of the down-scaling hypothesis, it is also not clear that these synaptic changes are in fact caused by sleep. There are a number of circadian changes in hormone release and even brain temperature that are independent of sleep, but known to alter synapses in ways ascribed to sleep. Disentangling circadian influences from sleep-wake influences is very difficult, but necessary before one can conclude that 'sleep' is doing something.

With respect to brain metabolism, this is an old idea about sleep, but also one that lacks 100% convincing support. There are conflicting reports on overall metabolic changes in the brain across the sleep-wake cycle. Some studies show that during sleep, brain metabolism is reduced, other studies don't show this. Likewise, while it seems intuitive that sleep is restoring something in the brain, what that something is also not 100% clear. Most changes in ATP metabolites are extremely short-lived, and the 'benefit' of sleep is basically gone within a few minutes of wakefulness when the 'surplus' is used. In addition, other energy stores in the brain (like glycogen) are not consistently depleted by extended wake, or can even restore themselves without sleep. The few studies that have shown ATP increases in sleep are somewhat controversial because measuring these molecules in the brain is very difficult and there are disagreements in the field. The fact is that although we feel 'refreshed' or 'restored' after sleep, these are subjective sensations that may not be related to what is actually going on during sleep.

The recent Science article showing that the rate of protein clearance is elevated during sleep is very interesting, but also touches an other very old idea about sleep, that sleep 'detoxifies' byproducts of the waking brain. What is interesting in this study is that protein clearance also happens while we are awake, just not as fast. It would be interesting to know what happens when you keep an animal awake for longer periods. Would the protein clearance then increase during wake? That is, as is true for other proposed roles for sleep, sleep may be sufficient but not necessary for this process.

So I think it is best to be cautious when one thinks about sleep function. There are very few new ideas about sleep. Many of the ideas and theories about sleep you hear today were discussed decades ago. The evidence for any one of then is also less certain when you get up close to it.

Perhaps the world record attempts have something to tell: http://en.wikipedia.org/wiki/Randy_Gardner_(record_holder)

A lecture by Euan Macphail in the 1990s reported that the high school science project student, Randy Gardner, responded in a somewhat less delirious manner than the DJ, Tom Rounds. Gardner showed a decline on one of the mini mental state tests, subtracting backwards from 100 - he suddenly could not remember was he was doing; a memory function that might fall under the umbrella of executive functions.

In response to the stress issue: good point, but stress has been controlled for in a number of animal and human studies of sleep and plasticity. This was a crippling confound in studies conducted in the 70's and 80's, but there are several studies since that have fairly convincingly eliminated this as a confound. Many human studies of sleep and memory consolidation use very small amounts of sleep deprivation, which do not increase plasma concentrations of stress hormones. Animal studies have also been done where changes in stress hormones are negligible or controlled. So, I think it might be time at least for some studies, to move past this concern. That is not to say that cycles in stress hormones do not alter synaptic strength. They do based on a number of studies in vivo and ex vivo. That is why the more recent studies have made a point to measure and/or control for this.

The approach you describe makes sense Marcos and I will eat my words in the presence of good data. Please could you direct me to best cases of relevant investigations? A bit of extra sleep is stress-free, which Walker's data shows seems to show has benefits.

The reasons may come from deep science as well as common thinking that more the complexities more is requirement of their maintenance. Evolutionary linkage to sleep requirement does not satisfies me much because the biorhythms and cycles of rests have been worked out at unicellular levels.

There have been reports where sleep is strongly related to memory consolidation. But commenting on Risto llmoniemi's curiosity, I also find it very intriguing and I hypothesize that Sleep increases memory consolidation more than in awake situations, because there is less back noise and other functions during sleep are at basal level. But, this is a speculation.

Human and mammals need sleep to carry out the restore energy and neurotransmitters in neurons and glia.My recent article in sleep medicine has illustrations to show how cardiorespiratory synchronization increases the potential in all CNS cells . This mechanism explains REM and SW sleep.

Evidence supporting the idea that sleep is beneficial for memory consolidation is not so strong as some people believe. Most of the positive results of studies using material learned prior to sleep onset can probably be ascribed to nonspecific effects like less interference during sleep. Thus increasing memory performance after sleep. Or (in turn) by stress or lowered attention caused by periods of prolonged wakefulness and sleep deprivation prior to memory testing. Which could impair consolidation and memory performance. Finally, its important to specify which type or memory is affected: procedural or declarative memory. I believe the reports are more positive for the first than second type of long term memory.

Dear Xingxu, to a greater extent I red many papers on sleep useful for long-term and short-term memory consolidation.

The primary function of sleep is restoration of energy and optimal functioning of neural networks. My recent article notes the observation that cardiorespiratory synchronization always correlates with slow wave sleep and it is related to increase in neuronal membrane potential. Further research is needed to clarify the relationship of energy storage by ATP in neurons ., glia by oxygenation with cardiorespiratory synchronization.

Why do mammals sleep? You need to look at the whole picture-----

Some of the most prominent reasons are listed below.

Mammals need sleep for the following reasons:

1. Sleep helps to boost the energy level of the body.

2. during sleep, your body can fight with some health problems, which is not possible when you are awake.

3. Memory stabilization is performed when you sleep.

4. See sleeping is a natural process- there are certain reactions that neurons will perform which it will not do when you are awake.

5. during sleep, many body functions are accelerated and decelerated- [ metabolism and catabolism]

6. you wonder why the brain signals change during sleep- it does brain cleaning work like "Ccleaner" in computer.

There are other issues that are under research.

Hope this answer helps the readers.

Best,

INDRAJIT MANDAL