The diurnal cycle regulates the work of every organism, manifesting itself at every hierarchical level, both at the micro level - molecule, cell - and at the macro level at the population level. Light-dark influences the biological clock via glutamatergic signals to the suprachiasmatic nucleus via the reticulo-hypothalamic tract. In the brain, as in the whole body, there is a cycle of events that affects the economy of neurotransmitters, hormones, as well as activity cycles and sleep. In addition, for solutions to the physical environment in which patients are treated, exposure to sunlight in the patients' rooms, as well as the spaces they are in, is an important factor in regulating the diurnal cycle.