Sleep Cycles 101

There are few things greater than waking up feeling refreshed and revitalized after a good night’s sleep. But while you were asleep, far more intricate processes were occurring to help keep you functioning at your best, and to ensure your optimal performance for the pressing day ahead.

Benefits of Sleep

Sleep has been shown to improve:

  • Restoration of muscle growth following exercise
  • Memory consolidation — the formation of long-term memories
  • Mood regulation
  • Immune functioning
  • Reduces risk for chronic illnesses associated with inadequate sleep (diabetes, cardiovascular disease, and obesity)
  • Regulation of metabolism
  • Problem-solving
  • Detoxification of oxidative radicals produced while awake

Understanding Your Sleep Cycle

There are five stages of sleep, which are categorized more broadly as rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. This cycling between NREM, REM and brief periods of arousal occur throughout the night. Entrance into the various stages of sleep is observed by differences in your brain wave activity, often measured by an electroencephalography, or EEG. Different brain wave patterns (alpha, beta, theta and delta waves) correspond to EEG patterns of waking and sleeping.

Typical sleep cycles lasts around 90 minutes; however, the amount of time that is spent in each stage can differ over the course of the night and can depend on the time of day we fall asleep (such as whether you’re a night owl or an early riser)!

sleep cycles

NREM Sleep (Stage I to IV)

As you begin to feel drowsy and doze off, you enter Stage I sleep, characterized by theta waves that are irregular and slow in frequency. As you drift into light sleep, or Stage II sleep, the brain waves continue to decrease in frequency and amplitude, with intermittent spurts of high-frequency waves known as “sleep spindles.” These sleep spindles represent short bursts of brain activity, lasting typically 1–2 seconds.

As you begin to fall into moderate and deep sleep you enter Stages III and IV, which are called Slow Wave Sleep (SWS). EEG waves continue to decrease in amplitude, until they reach low frequency delta waves in deep sleep. It is most difficult to wake someone who is in NREM sleep. 

SWS is said to be associated with helping consolidate our declarative memory, or the learning of facts and events. For example, remembering that you have a coffee date with a friend tomorrow, or remembering your new colleague’s name.

REM Sleep (Stage V)

REM sleep is found interspersed among cycles of NREM sleep and this is where most of our dreaming occurs. In this stage, your brain activity appears to be in a wakeful state, however, you still remain asleep. Your muscles are inhibited from moving, called sleep paralysis, likely to protect you from “acting out” your dreams such as punching a wall while you fight valiantly against your intergalactic foes! REM sleep is also associated with procedural memory consolidation, for learning skills such as how to ride a bike, learn tennis, or how to keep those yoga poses without falling over.

In summary, your sleep plays numerous restorative and protective roles, allowing your brain and body to repair any damages acquired during the day, retain and process your complex memories, and protect and refresh us for all the exciting activities ahead of us. 

 

References:

Kaplan 2015. MCAT Behavioural Science Review. Chapter 4: Cognition, Consciousness and Language.

https://www.ncbi.nlm.nih.gov/books/NBK10996/

https://www.livescience.com/43153-declarative-memory.html

https://www.livescience.com/43595-procedural-memory.html

Benefits of Sleep:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3256323/ (Immune Function)

https://www.ncbi.nlm.nih.gov/pubmed/25200984 (Mood)

https://www.smrv-journal.com/article/S1087-0792(15)00103-3/abstract (Immunity, Detoxification,

https://www.cdc.gov/sleep/about_sleep/chronic_disease.html (Chronic Illnesses)

https://www.karger.com/Article/Abstract/358858 (Metabolism)

https://www.ncbi.nlm.nih.gov/pubmed/23055117 (Problem Solving)

https://www.ncbi.nlm.nih.gov/pubmed/23371889 (Oxidative Stress and Cancer Risk)