The question arises of what exactly is the purpose of sleep. The short answer is that we don’t know exactly. However, as the science of sleep progresses, our understanding as to the true purpose of sleep will likely be elucidated.
The fact that in deep NREM sleep physical and mental activity slow down considerably, has led to the reasonable speculation that this phase of sleep serves as a recovery and recharging function. While lower species don’t have REM sleep, almost all species do have NREM suggesting that this phase of sleep has a restorative function. When you’re in NREM you’re recharging your batteries. More specifically, N3 sleep or deep sleep is felt to be the most restorative.
This recharging function is essential because a prime consideration of both the brain and body is energy production and conservation. The brain’s prime goal is survival and its primary defense system, the fight/flight system, uses a lot of energy. Thus the production and conservation of energy is a critical function and it is assumed that NREM fills this role. When people are deprived of NREM sleep they do continue to feel very tired and the body tries to compensate for this on subsequent sleep occasions.
If NREM is about energy conservation, what is REM? Paradoxically REM uses a lot of energy. Perhaps the NREM phase of recovery, mentioned above, is among other things, designed to ensure that there’s enough energy for the REM phase?
Some researchers have argued that because of the high energy output in REM, the energy conservation of sleep overall is pretty small, particularly compared to being awake and just resting. Others have pointed out that if restoration of energy was the purpose of sleep, large animals should require more sleep, but in fact they sleep less.
One view of the energy consuming REM phase of sleep is that its main function is the consolidation of memories formed during the day. This would be crucial for learning and indeed young children do seem to require more REM than adults. One metaphor that might be useful here is that during the REM phase of sleep, the brain is filing away the day’s events. As you open the relevant filing cabinet you will see other associated ideas and other relevant recent events and their associations. If that were the case, our dreams would involve recent events and associations these events have with past experiences. Of course, a lot of dreams are like that. Moreover, the dreamer’s emotional state at the time of the dream would likely shape the dream sequence. So it’s likely that some consolidation of memories occurs during REM sleep and this process helps to explain dreaming.
While the consolidation of memories might be an efficient use of sleep time, it doesn’t appear too critical to functioning. As mentioned above, many species don’t even have a REM sleep phase, and when REM is suppressed it doesn’t appear to affect basic functioning. However, in humans, when REM is suppressed for while, as in heavy alcohol consumption, there is a period after the REM suppression in withdrawal, during which there is increased amounts of REM sleep, what is called REM rebound.
The evidence is not overwhelming but it is reasonable to assume that NREM sleep serves some energy restoration function and that NREM sleep serves some neural consolidation of memories and learning.
Now we have a better understanding of sleep, it has allowed us to research sleep patterns and determine the type, rate and costs of poor quality sleep, reduced sleep time, or a combination of both. And we have been unable to expose the myth, common until about 20 years ago, that sleep deprivation or poor quality sleep doesn’t cause serious health and economic consequences.