Daytime sleepiness influenced by Vitamin D levels and skin pigmentation
Ah, happiness, that elusive state. Philosophers, theologians, psychologists, even economists, have long sought to define it, and since the 1990s, a whole branch of psychology—positive psychology—has been dedicated to pinning it down and propagating it.
Often referred to as the "body clock", the circadian rhythm is a cycle that tells our bodies when to sleep, rise, eat--regulating many physiological processes. This internal body clock is affected by environmental cues, like sunlight and temperature.