We know the REM sleep is characterised through it's rapid eye movements behind closed eyelids. At the same time we brain activity (through EEG) looks pretty similar compared to the wake resting periods and the individual experiences a lack of muscle tone. According to Simor et al., (in press) however the microstructure of REM sleep is heterogeneous. Especially when phasic and tonic REM stages are compared with regard to spontaneous and evoked cortical activity, information processing and mental experience.

But if phasic and tonic REM sleep differ in a range range or parameters why has REM sleep been looked at as homogenous by the vast majority of studies so far?

Simor, P., van der Wijk, G., Nobili, L., & Peigneux, P. (in press). The microstructure of REM sleep: why phasic and tonic? Sleep Medicine Reviews, 101305. https://doi.org/10.1016/j.smrv.2020.101305