Grades 3–5 · Organisms
How does your body know when to feel sleepy?
Feeling sleepy is not controlled by a single on-off switch. One influence is your daily body clock. Another is the need for sleep that generally builds while you are awake and decreases as you sleep.
In the diagramTwo processes help time sleepA daily clock and a growing need for sleepDaily body clockSleep needLonger time awakeLight through the eyes helps align the clockThese are processes, not a bedtime calculator.
Light reaching the eyes sends information to a central clock in the brain. This helps align the body’s roughly daily rhythm with the outside day and night. The clock helps coordinate many changes, including the evening rise of a hormone called melatonin. Melatonin is a timing signal, rather than a substance that simply switches the whole brain off.
The need for sleep and the clock can pull in different directions. That helps explain why a person can feel tired after a long time awake yet have difficulty sleeping at an unusual time. During sleep, the brain remains active and moves through different sleep stages. The picture separates the two timing influences so they are easier to understand; its clock and bars are not measurements of anyone’s sleep.
About this explanation
Starting knowledge: Know that the brain receives information from the eyes.
Sources
- National Heart, Lung, and Blood Institute — How Sleep Works: Your Sleep/Wake Cycle