The correct choice is early sleep, as this stage is specifically associated with the rising levels of melatonin, which plays a critical role in regulating sleep-wake cycles. During early sleep, the body initiates various physiological processes that prepare it for restorative sleep. This includes a natural drop in core body temperature, which helps to facilitate the onset of sleep.
Melatonin, often referred to as the “sleep hormone,” is produced by the pineal gland in response to darkness and signals the body to prepare for sleep. As melatonin levels rise, the body begins to decrease alertness and ready itself for deeper stages of sleep. The decrease in core body temperature is a physiological response linked to sleep onset; the body’s temperature naturally cools to help promote sleep.
In contrast, REM sleep is characterized by increased brain activity and is when most dreaming occurs, but it does not coincide with significant changes in melatonin or core temperature. Deep sleep is known for its restorative processes, with lower brain activity, but does not mark the initial physiological changes associated with sleep onset. Late sleep refers to the later hours of sleep when individuals may transition between sleep stages but is not specifically linked to the initial rise in melatonin and body temperature changes.