Sunlight helps the suprachiasmatic nucleus (SCN), a group of cells in the brain’s hypothalamus, keep the human circadian rhythm in sync with the sun. This includes resetting this 24-hour biological clock on a regular basis. The process, known as entrainment, occurs when light-sensitive cells in the retina send electrical signals to the SCN. In humans, at sundown when the brain registers that there is no more sunlight outside, the SCN tells the pineal gland to release the hormone melatonin, which helps to promote a sense of being sleepy. (If the brain is exposed to artificial light after sundown, including LED lights used in most electronics, the release of melatonin can be interrupted). In the morning, as sunlight enters the eyes, the SCN is activated and wakes up the body organs, notifying the pineal gland to stop secreting melatonin. If natural light cannot get to the retina, the cycle of the circadian clock begins to lengthen beyond the usual 24-hours and a few minutes, which can be disruptive to a person’s life.