Final answer:
The sleep-wake cycle is regulated by the suprachiasmatic nucleus in the hypothalamus, which receives signals from the eyes and helps synchronize our internal clock with the outside world.
Step-by-step explanation:
The sleep-wake cycle, also known as the circadian rhythm, is regulated primarily by the suprachiasmatic nucleus (SCN) in the hypothalamus. The suprachiasmatic nucleus receives signals from the eyes in response to light levels and helps synchronize our internal clock with the outside world. It sends signals to the pineal gland, which produces melatonin, a hormone that helps regulate sleep.