Sleep is an incredibly elusive concept in the scientific community. While neuroscientists understand a lot about sleep, the question still remains, why? Why do humans need sleep to survive and what purpose does it serve? While we may not be able to answer this question quite yet, we can examine what we do know about sleep!
For many years, scientists thought that all brain activity ceased during sleep, citing the loss of conscious experience. Today, however, we know that the brain remains incredibly active during sleep (Hobson, 2005)!
Scientists often categorize sleep cycles based on the types of brain waves seen in electrical recordings. Each sleep cycle lasts approximately 90 minutes and occurs in this order:
Awake → N1 → N2 → N3 → N2 → REM sleep
When we are awake, our brains produce high frequency low amplitude waves called beta waves. If you get into a drowsy/relaxed state, your brain begins to produce lower frequency alpha waves as you doze off. Once you are asleep you enter non-rem stage I (N1). During N1, alpha waves decrease and are replaced by slightly lower frequency electrical signals called theta waves. This generally lasts for about 5 minutes, before progressing into N2. In N2 the theta waves are accompanied by short bursts of K-complexes and sleep spindles (lower frequency waves) that show the progression toward even slower wave sleep. This stage generally lasts for about 25 minutes (though it lengthens with each additional cycle). The last non-rem stage, N3, is often referred to as “slow wave sleep.” At this point, the brain produces very low frequency delta waves, and this stage lasts for approximately 20-40 minutes. The brain waves then begin to increase in frequency, briefly returning to N2, before beginning rapid eye movement sleep (REM sleep). REM sleep occurs for 10 minutes to an hour (increases with each cycle) (Patel et al., 2021; Purves et al., 2011; University of Michigan, 2020; Cleveland Clinic, 2020; Scientific American, 1997).
As scientists continue to push the boundaries of our sleep knowledge. We will continue to gain more insight into the elusive function of sleep.