Final answer:
The hormone that controls the brain's natural sleep cycle is melatonin option (a), which is secreted by the pineal gland in the brain. It helps regulate the sleep-wake cycle by responding to light and dark and is closely linked to the body's circadian rhythms managed by the suprachiasmatic nucleus.
Step-by-step explanation:
The brain's natural sleep cycle is controlled by a hormone known as melatonin. The pineal gland, an endocrine gland located in the brain, secretes melatonin in response to low light levels, playing a crucial role in regulating the sleep-wake cycle.
During the day, when light stimulates the retinas, the production of melatonin is inhibited, leading to wakefulness. In contrast, as it becomes dark in the evening, melatonin production ramps up, increasing blood levels and inducing drowsiness, thus promoting sleep.
This process is connected to the suprachiasmatic nucleus (SCN) within the hypothalamus, which is considered the body's biological clock. The SCN helps to regulate biological rhythms, including the circadian rhythm that dictates our sleep-wake patterns. Exposure to light and darkness is a key environmental cue that the SCN uses to help synchronize our circadian rhythms with the external world.
The circadian rhythm disruptions such as those experienced during jet lag or shift work can interfere with normal melatonin production, leading to problems with sleep. With adequate exposure to natural light during the day and darkness at night, melatonin levels can help maintain a healthy sleep cycle, contributing to overall well-being.