Why Do We Feel Sleepy At Night

Explains Biplab Das

What precisely triggers sleepiness is still a mystery. But, available clues centre around the circadian clock inside our body. In humans, a circadian clock runs for about 24.5 hours and the centre that controls this clock lies inside the brain. The clock is activated by the intensity of light around us. Preempting tired brain and the darkness of night, the clock lulls the fatigued neurons. This results in the reduction of blood flow from brain, ushering in sleepiness. 

Circadian clock also alerts the brain to warm up a few hours before the day begins. Brain's cerebrum goes through day-to-day wear and tear and needs to be repaired. 

It is during sleep when the brain's recovery process sets in. At the outset core sleep engulfs us. Then, after a few hours, core sleep subsides letting optional sleep to continue. Circadian rhythm controls this optional sleep, switching in at night and lifting by day. 

All our physiological activities have circadian rhythms, which somehow come into play during the onset of sleep. A hormone called melatonin is also believed to be related to nighttime sleepiness. Daylight inhibits the hormone's release and at night its levels rise dramatically.