The gland that reports the dark
Deep between the two halves of the brain sits the pineal gland, a pea-sized structure that once led philosophers to call it the seat of the soul. Its real job is humbler and more elegant: it converts information about light and dark into a chemical signal. At night it secretes melatonin, a neurohormone that tells every tissue in the body, in plain chemical language, that it is dark outside.
Melatonin rises a couple of hours before your usual bedtime, peaks in the middle of the night, and falls toward morning. It is not a knockout drug — it does not force sleep the way a sedative does. It is more like a dimming of the lights, a signal that lowers alertness and opens a window in which sleep comes easily. Its level is a faithful diurnal marker of the body's sense of night.
Who tells the pineal when it is night
The pineal gland does not see light itself. It takes orders from the body's master clock, the suprachiasmatic nucleus (SCN), a small cluster of neurons in the hypothalamus. The SCN is the biological clock that sets the daily, roughly 24-hour circadian rhythm for the whole body. Left alone in a cave, the clock keeps ticking, but it would slowly drift off the true day.
To stay accurate the clock needs to be reset every day, a process called entrainment. The strongest resetting cue is light. Special light-sensing cells in the eye send signals to the SCN; bright morning light nudges the clock earlier, light at night nudges it later. The SCN then relays the verdict to the pineal — and only when the SCN says it is truly dark will the pineal release melatonin.