Sleep consists of Rapid Eye Movements (REM) and Non Rapid Eye Movements (NREM) sleep which alternate in periods of 90-100 minutes depending on circadian rhythm. 5
REM Sleep
REM sleep was investigated in 8 healthy young males deprived of sleep for 3 consecutive nights. Each consecutive night it became easier and easier for the males to fall into REM sleep however, to maintain sleep deprivation, the subjects were awakened every time they almost fell asleep. This study found that the necessity for REM sleep increases with increasing sleep deprivation. REM sleep is responsible for our ability to dream while we sleep. 5
Non-REM Sleep
There are three stages to NREM sleep that occur prior to reaching REM sleep. The first phase is when your eyes are closed but you can still be woken up easily. This phase lasts for about 10-15 minutes. The second phase is light sleep when your body prepares for deep sleep. In this phase, your heart rate slows down and your temperature drops. Lastly, your body enters deep sleep. 6
References
5. Endo T, Roth C, Landolt HP, et al. Selective REM sleep deprivation in humans: effects on sleep and sleep EEG. Am J Physiol. 1998;274(4 Pt 2):R1186-94. 6. Peszka J, Harsh J. Effect of sleep deprivation on NREM sleep ERPs and related activity at sleep onset. Int J Psychophysiol. 2002;46(3):275-86.