The only known function of sleep is to cure sleepiness.
Skipping out on sleep has a lot of harmful effects, from grouchiness to obesity to a huge coffee bill. But why do we need sleep, and why do we need so much of it? It turns out, opinions are divided. The quote leading this article may be the most solid explanation available – we sleep because we’re tired. But there are some better ideas. Four major theories have dominated sleep study:
1: Inactivity Theory
Inactivity is one of the first theories about the necessity of sleep, and has also gone by names such as Adaptive Theory or Evolutionary Theory. Inactivity Theory states that there is an evolutionary advantage to sleep – that sleep helps people survive.
By staying quite and still during the dark hours of the nights, individuals remain out of harm’s way: and out of the jaws of a predator. Sleepers are also avoiding possible accidents and other risks that affect mobile creatures. By increasing their time spent asleep, they survive longer, and thus sleep becomes an asset.
However, this theory is not widely accepted, and has a number of flaws. Most prevalent is the common-sense test – if a creature is asleep, they are not able to respond to potential threats. An awake mouse is better able to flee a cat than a sleeping one.
2: Energy Conservation Theory
The next is Energy Conservation Theory, and as the name suggests has to do with sleep saving an individual’s energy. Compare two creatures, one that is active 24 hours a day, and one that is active for 16, and asleep for 8. The theory holds that the sleeper is better able to survive in times of scarce food, because they use less of it.
Essentially, with Energy Conservation Theory animals trade time for energy. They have less time awake, but because of that, they use less energy.
Part of this is true – when sleeping, animals and people do use fewer calories. But the amount reduced is not so much as you may think.
When humans sleep, energy metabolism is reduced by about 10%, and the body temperature drops It’s doubtful that this kind of reduction would make a significant change in survival and overall energy consumption rates.
3: Restorative Theories
The third theory is one of the most popular, and has several versions. Generally speaking, Restorative Theories assert that sleep “restores” something that is spent while we are awake. Sleep is a time to repair and rejuvenate the body and mind.
There is some credence to these theories, and lab testing to prove it out. In some studies, animals that were deprived of sleep eventually lost all their immune function, and within a few weeks had died.
As well as immune function, sleep is also a time for:
- Muscle growth
- Tissue repair
- Protein synthesis
- Growth hormone release
These vital body activities occur primarily during sleep.
As we can all relate, sleep is not just about the body, but the mind too. Sleep has an important function with relation to a brain chemical called adenosine. Adenosine is a by-product of regular cellular activity. Unlike some products, like carbon dioxide, it is not regularly expelled from the body. Instead, adenosine builds up in the brain over the day. This accumulation of adenosine may be one of the key properties that contribute to feeling tired. During sleep, the adenosine is cleared and the tiredness feeling goes away.
(Sidenote: Adenosine’s behavior is counteracted by caffeine!)
4: Brain Plasticity Theory
Brain Plasticity is one of the more recent theories around sleep. It holds that sleep is a time for changes to the structure and organization of the brain. Recent brain research has shown that the brain is not static, and can change and adapt over time. Sleep in particular is when this can take place.
Brain Plasticity plays a big role for learning and memorization.
While you sleep, your brain does not rest idle. Instead, your brain reviews and sorts the activities and information absorbed through the day. When you learn something, you may not really “know” it until you’ve slept on it. This could be why when learning a new ability, like skating, you may struggle with a particular move. After sleeping, the brain has a chance to review the action and understand it. The next day, you can do it.
This applies not just for activities, but all kinds of memorization – sleep before, and after learning helps your brain make sense of the new sensory experiences. It codes them into a firm, long-term form. This method of solidification of memory also creates new links between memories. Rather than recalling our activities as one chain of events in time, we can think of things conceptually. So my skating shoes are associated with not just skating, but also that they are my favorite color, the time I fell on the ice, skates I wanted but couldn’t afford, etc. etc.
Sleep helps make life into memory, and experiences into learning. The effect of sleep on memory can’t be denied – a study in Neuropsychology submitted test subjects to extreme sleep deprivation – four days with almost no sleep. At the end of the study, the average working memory has been reduced by almost 40%.
These reasons account for the fact that newborn babies sleep for a large amount of their lives – upwards of 13 hours every day. Half of that time is spent in deep, REM sleep, compared to no more than 25% for adults. As babies are learning about the world around them, they need more deep sleep to process that information. This latter element can be grouped into a theory of its own – Ontogenesis, which states that deep REM sleep is a function of developing the brains of new born life.
Image credit: Bebe, maternidad subrogada by Maternidad Subrogada – Subrogalia, on Flickr