There is considerable diurnal variation in renal function. Water and sodium are retained at night and of the 1–2l of urine that is normally excreted in the 24h, around 80% is produced during the day.
The changes in immune function in sleep are less well documented than the autonomic changes, but the blood eosinophil and cortisol levels, and natural killer (NK) cell activity fluctuate with a circadian pattern.
There is a relationship between melatonin, cytokines and interferon, all of which have immunological actions and influence sleep. The T-lymphocyte level rises in sleep, but the peripheral blood monocyte concentration falls, largely because the cells are sequestered in the spleen. In addition, changes in autonomic activity regulate the local immune responses through, for instance, alterations in regional blood flow. Peripheral release of cytokines may also affect sleep through stimulation of vagal afferents which reach the brain-stem, as well as through direct transfer across the blood–brain barrier.