The function of sleep

Despite the numerous attempts to attribute a specific function to sleep, scientists are still looking for an answer to the question "why do we sleep?". Sleep seems to be a type of behaviour during which functions considered essential, or at least useful, for our nervous system and our body are carried out. If this were not so, there would be no explanation for the fact that sleeping has been preserved throughout evolution.
Although representing a period when the body’s senses and movements are cut off from its external surroundings, sleep is characterized by a continuous cerebral activity. Furthermore, the metabolism of the brain is only very slightly reduced during NREM

sleep and returns to levels typical of the waking state during REM

sleep. The brain, therefore, does not remain inactive during sleep. The effects of sleep deprivation have been studied in animals: a rat, totally deprived of sleep for about one month (Fig. 1, red line), dies due to a serious metabolic syndrome characterized by an incontrollable drop in body temperature and weight. It is impossible to make up for this loss by administering food to boost energy levels. Many doubts remain over the specifics of this effect, given that the animal is under great stress due to the deprivation procedure. However, it has been seen that an animal subjected to similar levels of stress but which is allowed 60-70% of its sleep shows much less evident pathalogical signs (Fig. 1, blue line).

Fig. 1: Progress of a laboratory rat regarding food consumed, energy expenditure and body weight over the course of one month under two experimental conditions: a) total deprivation of sleep (red line); b) partial deprivation of sleep (blue line).
(Credit: G.Tononi, C.Cirelli. Sonno. Da "Fisiologia Medica". Edi.Ermes, 2005)

There are many theories on the function of sleep. Some of these concern the possibility that certain cerebral metabolic functions of a specific nature are activated during sleep to make up for physiological modifications that occur during waking hours, but experimental data has not provided us with conclusive answers so far. Other theories emphasize the role of sleep in favouring the phenomena of plasticity in nerve cells. In this regard, the data collected so far points to a determining role for sleep in modelling and stabilizing new contacts between nerve cells. These contacts would be crucial for consolidating the remembering of information acquired during waking hours. Nevertheless, despite researchers’ best efforts, the function of sleep still remains a mystery

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