What is sleep?

Contrary to popular belief, sleep is not a passive state where the body and mind are in ‘shut down’ or ‘reboot’ mode. Sleep is an altered physiological state necessary for the proper functioning of our body and brain. Although we are disconnected from the outside world, sleep is an active state, where the brain transitions through different phases of activity that can be separated into different sleep stages.

Illustration of electrophysiological activity of the brain (EEG), eyes (EOG) and muscles (EMG) throughout the different sleep stages.

The more commonly known of the sleep stages is rapid eye movement sleep or REM sleep. It makes up about 20-25% of the adult’s sleep and is more prominent in the later part of the night. As its name indicates, and as shown above, this part of sleep is characterized by the presence of eye movements. Although the eyes are closed, they move back and forth in a saccadic fashion. At the same time, the brain is active during REM sleep in a pattern that is similar to that of relaxed wakefulness. REM sleep is also associated with dreaming and dream recall.

Using very original nomenclature, the other sleep stages were named Non-REM sleep stages (or NREM sleep), which can be further divided into 3 stages: N1, N2, and N3. Throughout these sleep stages, sleep progressively deepens as the brain activity becomes more synchronous, completely shutting out from the outside world. N1 sleep is typically associated with the transition from wake to sleep and makes up about 5% of the sleep period in a healthy adult. N2 sleep is the most common of the sleep stages throughout the night, making up about 55% of sleep.  N3 sleep is the deepest of the sleep stages. It is characterized by slow waved activity in the brain, which plays an important role in restoration. Adults have approximately 15-20% of this sleep stage throughout, with the greater majority of this sleep found in the first half of the night.

NREM and REM sleep cycle throughout the night, with each cycle lasting approximately 90 minutes. This pattern is generally similar from night to night, however, there a number of factors that can influence each of these different parts of your sleep. Factors such as fatigue, sleep deprivation, circadian influences, medical conditions, etc. can play a big role on the quality of your sleep throughout the night.

Selected Supporting References:

  • Carskadon, M. A., & Dement, W. C. (2005). Normal human sleep: An overview. In M.H. Kryger, T. Roth, & W.C. Dement (Eds.), Principles and Practice of Sleep Medicine, 4th edition, (pp 13-23). St. Louis: Elsevier Saunders.
  • Aserinsky, E., & Kleitman, N. (1955). Two types of ocular motility occurring in sleep. Journal of Applied Physiology, 8(1), 1-10.
  • Dement, W., & Kleitman, N. (1957). Cyclic variations in EEG during sleep and their relation to eye movements, body motility, and dreaming. Electroencephalography and Clinical Neurophysiology, 9(4), 673-690.
  • Nielsen, T. (2010). Ultradian, circadian, and sleep-dependent features of dreaming. In M.H. Kryger, T. Roth, & W.C. Dement (Eds.), Principles and practice of sleep medicine, 5th edition, (pp 576-584). St. Louis: Elsevier Saunders.
  • Iber, C., Ancoli-Israel, S., Chesson, A., & Quan, S. F. (2007). The AASM manual for the scoring of sleep and associated events: Rules, terminology and technical specifications (Vol. 1). Westchester, IL: American Academy of Sleep Medicine.
  • Berry, R. B., Brooks, R., Gamaldo, C. E. et al. for the American Academy of Sleep Medicine. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications, Version 2.2. American Academy of Sleep Medicine, Darien, IL, 2015.
  • Aubin, S. (2018). L’influence de la cécité sur le rythme circadien et le sommeil.

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