Sleep is a type of behaviour generated by mechanisms located in the central nervous system. The nerve centres mainly held responsible for the physiological signs of sleep are found in the diencephalon and in the trunk of the encephalon. These structures form the deepest and, from an evolutionary point of view, the oldest parts of the encephalon (fig. 1).

Fig. 1: In the encephalon above we can see: a) ) the rostral part (IR) and the caudal part (IC) of the hypothalamus, a structure situated at the base of the part of the encephalon known as the diencephalon, b) the back part of the trunk of the encephalon (TED) and the sostanza reticolare attivante (SRA), which is located in the trunk of the encephalon and continues in the caudal hypothalamus. (Credit: G.Tononi, C.Cirelli. Sonno. In "Fisiologia Medica". Edi. Ermes., 2005.)

It has been known for some time that sleepiness and insomnia in humans can be caused by damage in the caudal (IC) and rostral (IR) parts of the hypothalamus respectively, as observed by von Economo in patients affected by encephalitis following the so-called "Spanish flu" pandemic.

On the other hand, movements characterised by REM sleep, such as muscular hypotony, sudden muscle contractions (mioclonie) and rapid eye movements, depend on the activity of nerve centres situated at the back of the trunk of the encephalon (TED).

The waking state, instead, is favoured by the activity of various nerve centres distributed along the trunk of the encephalon, in a structure called the sostanza reticolare attivante (SRA), which extends up as far as the caudal hypothalamus (IC).