A quick answer to that question is that most people work in the daytime when it’s light. They, therefore, feel most tired after work when it begins to get dark, particularly in the winter months. A scientific answer is that when the retina in the eye senses light, or it getting lighter in the morning, it passes a message to an area of the brain known as the hypothalamus. That area is referred to as the suprachiasmatic nucleus (SCN). This nucleus activates the pineal gland in the brain which in turn raises your body temperature and promotes the release of hormones such as cortisol, that stimulates the body into wakefulness. So when it gets light, your brain wakes your body up. Conversely, when it gets dark, the SCN switches the pineal from emitting cortisol to releasing melatonin into your bloodstream. This is a hormone that makes you feel sleepy. So, the SCN is informed when it is light and is time to waken you, and when it is dark and is time to promote sleep.