The Circadian rhythm … our natural 24 hour sleep wake cycle:

So light affects our sleep cycle? Yes- light, especially light from the blue part of the spectrum is captured by specialized cells on the retina in the back of the eyes, and travels to a part of the brain within the hypothalamus called the Suprachiasmatic Nuleus (SCN for short). Light is the trigger for turning on many biological processes in the body including turning on production of hormones such as cortisol (the body’s own steroids), adjusting body temperature, and many other functions. Light helps us wake up and light can be used to help adjust our sleep wake cycle if it becomes disrupted. Jet lag and shift work affects our sleep cycles, and light is used to help us get back on track. Bright lights in the work place for night workers helps them stay awake, and early morning sunlight helps travelers suffering from jet lag when travelling from the US to Europe for example. Teenagers have disruptions in their normal circadian rhythms causing them to go to sleep very late and wake up very late (if they can … on week-ends). Bright light in the morning, and dim light in the evenings can help with that.