How do light & dark affect us? Each of us has a 24-hour biological clock in the brain's hypothalamus region. Known as our circadian rhythm, the amount of light we absorb in a day impacts this region, negatively or positively affecting things like our sleep, mood, digestion, temperature control, energy levels and cell renewal. When winter comes around and the days get shorter, the lack of exposure to sunlight can stop the hypothalamus from working properly, affecting: - Production of melanin - Production of serotonin - Circadian rhythm Which can lead to SAD – seasonal affective disorder. Check out the link in our bio for the full blog post. *Graph shows fluctuations of cortisol, melatonin and body temperature at different times of the day.