All You Need to Know About Our Sleep Architecture: Sleep Stages, Types and Asymmetric Cycles

Different Types of Sleep

Research indicates that during the night while asleep, people cycle through two completely different types of sleep. These are rapid eye movement or REM sleep and non-rapid eye movement or NREM sleep. During REM sleep our brain activity is very similar to being awake. This is when we are dreaming, therefore often described as dream sleep. The brain wave activity during NREM sleep is much lower and is subdivided into four separate stages. The first two stages of NREM sleep can be described as “light sleep”. Furthermore, stages 3 and 4 are the “deepest stages” of NREM sleep. Each sleep stage has a different role in allowing your mind and body to wake up refreshed.

Time Distortion During Sleep

Interestingly, during NREM sleep you lose all awareness of time. On the other hand, during REM sleep you continue to have a sense of time. It is however not particularly accurate as dream time is stretched out and prolonged relative to real time. The reason for this is because during REM sleep, memories and experiences from the previous day(s) are being “replayed” at half or a quarter of the speed. This is why when you snooze your alarm in the morning and you dream for 5 more minutes it felt like you were dreaming for much longer.

Sleep Cycles Throughout the Night

Throughout the night a person goes through different stages of sleep as part of multiple sleep cycles. Each sleep cycle takes around ninety minutes. As seen in the illustration below, the ratio of NREM and REM sleep within each ninety-minute cycle varies significantly throughout the night. During the first half of the night our ninety-minute sleep cycles contain an abundant amount of NREM sleep and very little REM sleep. However, in the second half of the night our sleep cycles are largely dominated by REM sleep with little if any deep NREM sleep (NREM stages 3 and 4). Typically, a person goes through four to six sleep cycles per night.

Explanation For the Asymmetry of Sleep Cycles 

There is no proven explanation for the asymmetric sleep pattern throughout the night. One theory suggests this is because the key function of NREM sleep is removing unnecessary neural connections. Furthermore, REM sleep plays an important role in strengthening neural connections which are deemed important. Consequently, this would explain the cycling nature of NREM and REM sleep and the imbalance of their distribution throughout the night. This because this is necessary in order to efficiently manage the memory storage system in our brain.

The Evolutionary Benefit of a High Amount of REM Sleep For Humans

Between 20 and 25 percent of our sleep time at night is dedicated to REM sleep dreaming. This is the most of all animals and significantly more than the average of 9 percent REM sleep across all other primates. A key functions of REM sleep is the recalibration and fine tuning of the emotional circuits of the human brain. The bigger amount of REM sleep resulted in a steep increase in our cognitive creativity, emotional intelligence, and social complexity. Therefore, it is likely that the high amount of REM-sleep dreaming of humans represents an important factor in the process of our astonishingly rapid evolutionary rise to power.