As the seasons change, so does the amount of sunlight each day, which in turn causes changes in people's internal biological clocks (also called circadian rhythm). This rhythm is a 24-hour cycle that affects our eating and sleeping patterns, as well as biological activities such as brain wave activity, hormone production, and cell regeneration.
In some individuals, less daily sunlight and changes in circadian rhythm can induce depression, which usually peaks in January and February, when there is the least sunlight. This is called seasonal affective disorder (SAD) or seasonal depression. Researchers believe that the lack of sunlight during these times may alter brain levels of certain mood-controlling substances—for example, the hormone melatonin, which may decrease.