This gland is situated in the middle of the brain, around a quarter or an
inch long and secretes a hormone called melatonin when it’s dark that
regulates the sleep – wake cycle. This is why some people feel depressed
in the dark winter months when they may be producing too little
melatonin. The pineal gland is shaped like a pine cone and helps to
promote sleep as it is sensitive to light. But can also affect reproduction
by depressing the activity of the gonads.