|Everyone sleeps. Image by PedroSimoes7|
Role in other processes
Sleep can affect a number of areas of our waking lives, including:
- Education: early risers get better school grades (Preckel et al., 2013)
- Memory: sleep plays a major role in consolidating memories (Walker & Stickgold, 2004)
- Creativity: people with later sleep cycles are more creative.
- Mood: an afternoon nap can tune out negative emotions (Gujar et al., 2010)
- Metal health: sleep problems are associated with stress and depression.
When people don't sleep, they lose the ability to concentrate on even the simplest of tasks, and begin to hallucinate and feel deeply paranoid (Lindzay et al., 1976). It is also so unpleasant that sleep deprivation is used as a form of torture.
What happens during sleep?
Activity in the brain changes over 5 sleep stages. The first four of these often just called stages 1-4, and during these, you fall into a progressively deeper sleep. In stage 5, 'REM sleep', the brain becomes more active again, and you begin to dream - something that Freud considered to be focused on 'wish fulfilment'.
Electrical activity in the brain changes over the sleep stages. One particular brain wave, the delta wave, has a very slow frequency and a high voltage, and is only normally found during deep sleep. There are no delta waves in stages 1 or 2. During the first stage, the brain’s activity is very similar to wakefulness with small rapid brain waves, but by stage 4, delta waves are dominating.
Because of this, stages 3-4 are often called delta sleep or slow-wave sleep (SWS), and it may be the case that these stages are especially important to allow the brain to consolidate episodic memories, and allow the brain to recover in order to store new memories the next day (Walker, 2009).
The evolution of sleep
The evolution of sleep is a puzzle, but we know that as it exists in animals as basic as the fruit fly, sleep must have served a useful function throughout our evolution.
|All animals sleep; sloths sleep in the trees |
for large parts of the day. Image by Brian Gratwicke.
On the surface, sleep might appear to be a waste of valuable hours in the day, as well as leaving animals vulnerable to attack. For this reason, sleep must be a valuable - perhaps essential - process, and must fulfil a function for which rest would not be enough.
One possibility is that sleep evolved because it helped our ancestors to repair damage to the body, giving them a survival advantage. However it would appear that resting would fulfil the same function, and sleep deprivation does not seem to cause much immediate physical harm to individuals.
Another suggestion is that sleep is essential for the nervous system. Resting does not cut the individual's nervous system off from external stimulation in the same way as sleep does, and this could be its key purpose (Cirelli and Tononi, 2008). It may well be that as organisms developed more complex brains, sleep evolved as a means of consolidating memories and restoring cognitive functions.
- More on theories of sleep and dreams to follow....
Cirelli C, Tononi G. Is sleep essential? PLoS Biol. 2008;6:1605–11.
Gujar, N., McDonald, S., Nishida, M., and Walker, M. (2010). A Role for REM Sleep in recalibrating the sensitivity of the human brain to specific emotions. Cerebral Cortex, 21(1), 115-123. doi: 10.1093/cercor/bhq064
Lindsey, G., Hall, C.S. and Thompson, R.F. (1978). Psychology (2nd Ed.) New York: Worth Publishers.
Preckel, F., Lipnevich, A., Boehme, K., Brandner, L., Georgi, K., Könen, T., Mursin, K., and Roberts, R. (2013). Morningness-eveningness and educational outcomes: the lark has an advantage over the owl at high school. British Journal of Educational Psychology, 83(1), 114-134. doi: 10.1111/j.2044-8279.2011.02059.x
Walker, M.P. (2009). The role of slow wave sleep in memory processing. Journal of Clinical Sleep Medicine, 5(2 Suppl), S20.
Walker M.P. and Stickgold, R. (2004). Sleep-dependent learning and memory consolidation. Neuron, 30;44(1):121–33.