[单选题]Choose the best topic sentence from the four options below.
In most animals and humans，the desire to sleep is brought on by secretion of a hormone called melatonin. Melatonin is produced in a tiny gland known as the pineal gland. In the evening，the pineal gland reacts to the diminishing levels of daylight and starts to produce melatonin，which is then released into the blood and flows through the body making us drowsy. Its secretion peaks in the middle of the night during our heaviest hours of sleep. In the morning，bright light shining through the eye reaches the pineal gland which reacts by switching off the production of melatonin，thus removing the desire to sleep.