In detail

Sleep functions

Sleep functions

The fact that the dream has persisted throughout evolution and that we spend a third of our life sleeping, shows us that it is very important functionally. But the nature and functions of the dream are still the subject of many discussions and different hypotheses have been proposed that can become complementary to each other.


  • 1 What happens when we don't sleep?
  • 2 Energy conservation
  • 3 A restorative process
  • 4 The adaptation
  • 5 Cognitive Functions

What happens when we don't sleep?

The different experiments with sleep deprivation indicate that this should have an important role in the homeostatic body system, temperature control and balance of the immune system and that it probably participates in cognitive functions.

For example, it has been observed that rats deprived of sleep for 3-4 weeks, end up dying; if they are deprived only of REM sleep, survive twice as long. At the beginning of the deprivation they eat more than normal, it is assumed that it is not to lose weight, but still they do not recover the caloric balance, then they lose control of the temperature and the immune system is also weakened.

In any case, the fact that we find sleep as a phenomenon in all species of mammals and birds, indicates that, at least in part, it is physiologically necessary.

The effects of sleep deprivation do not seem to be as drastic in humans as in animals. One of the most spectacular cases described in humans is the case of a boy who spent 11 days without sleep. The symptoms that appeared progressively were bad mood, dizziness and nausea, memory problems, feeling tired and trembling. When he decided to sleep, he did it for fifteen hours in a row, then he waited twenty-three hours awake, to go back to sleep for ten hours. Against all odds, no side effects appeared and after a week his sleep rhythm was completely normal.

Energy conservation

When we sleep we spend less energy, since when we are in the SUN (slow wave sleep), there is a decrease in blood pressure, heart rate, respiratory rate, muscle tone, body temperature, etc. All this suggests that the possible dream's role would be energy conservation.

The idea that sleep forces rest is supported by the fact that small mammals sleep much longer, proportionally, compared to large mammals. They are animals with a high demand for energy for thermoregulation and locomotion, but with few reserves.

Comparison of the amounts of sleep between different species.

But why if you can rest during wakefulness without losing attention or vigilance, it is not done? How do we explain that during REM sleep there is a high metabolic expense?

A restorative process

The metabolic rate during sleep represents 15% of what we keep awake. The reduction in metabolic processes is also related to the characteristic decrease in body temperature.

However, sleep deprivation studies have not provided convincing evidence that sleep is necessary to maintain a normal functioning of the organism, since in these studies an important stress response of the organism is not observed. Also, on the one hand, Sleep deprivation does not hinder people's ability to perform physical exercise.; and on the other, we have that physical exercise does not make the person sleep more hours (although it may sleep earlier).

The sleep would be a process in which there are different anabolic processes that try to compensate for the physical and emotional wear of wakefulness; This would be a vital physiological need, we sleep because we need to rest and regain strength. The fact that during some periods of sleep occurs hormone release, I could support this hypothesis.

The adaptation

According to this, we sleep to avoid problems in periods when the species is more vulnerable. Based on this hypothesis, 1969 F. Snyder suggested that REM sleep could be a period in which there is more brain activation, in case you have to be alert to possible dangers.

Some species, in order to avoid possible dangers of getting into sleep, have developed a different sleep pattern for each cerebral hemisphere. This is the case, for example, of the dolphin Tursiops truncatus, in which each hemisphere sleeps separately.

But why does the dream come about in some species that would be safer if they never slept?

Sample of sleep patterns by hemisphere of the dolphin Tursiops truncatus. We can see how in the left hemisphere it has a record EEG of slow waves and, on the other hand, in the right we have a desynchronized activity typical of the day before.

Cognitive functions

Humans show few physiological deficits after a few days of deprivation; instead yes intellectual abilities are altered. After a period of sleep deprivation may appear perceptual alterations and hallucinations.

After prolonged periods of sleep deprivation, the subjects show a greater amount of REM and, above all, SOL sleep.

Sleep seems necessary for normal brain functioning.

Many studies, especially with animals, have studied the relationship between REM sleep and learning and memory processes, especially from the following observations:

  • Existence of cortical desynchronization
  • Emergence of the hippocampal rhythm
  • Increased blood flow
  • Increase in protein synthesis
  • Some phylogenetic correlation between REM sleep and the learning ability of species.
  • Existence of significant correlations between REM sleep and the learning capacity and memory of the subjects.
  • Activation during REM sleep of structures with a strong affective memory component, such as amygdala.

In an experiment conducted in 1994 by Karni and others, it was observed that REM sleep deprivation after a learning session involved poor homework retention the next day. On the other hand, the deprivation of the SOL did not produce these deteriorating effects.

In another experiment it has also been shown that brain patterns that are recorded in the hippocampus of rats that have traveled a maze during the day (they were rewarded with food), they reproduce very accurately when the rats sleep. This relationship becomes so precise that researchers could tell where in the maze the animal would be if it were awake.

Some authors, such as F. Crick, believe that REM sleep would allow a memory removal of information that is not useful to us, which we do not need to retain. Instead, other authors believe that this would be a function of the SOL. It may also be that each period contributes differently to the learning and memory processes.

What is certain is that the dream seems that physiologically it is necessary to survive and for a good brain functioning.