Many biological processes have a cyclical character. This applies, in particular, to the alternation of wakefulness and sleep, which under normal conditions is characterized by a certain stereotypy, but it can be disrupted—for example, during long flights with changes in time zones, or in connection with work involving periodic night shifts, as well as in some extreme situations.
Changes in the level of wakefulness (for example, overexcitation, decreased attention, manifestations of asthenic syndrome) as well as sleep disorders (excess or insufficient sleep, disturbances in sleep quality) can have a very significant effect on a person’s activity, ability to work, and overall mental and physical state.