Reduced vitamin D intake
Roughly 1 billion people worldwide are vitamin D deficient. Sunlight is an essential source of vitamin D, but during the winter, people usually spend less time outside in direct sunlight. In adults, symptoms of vitamin D deficiency include fatigue, muscle weakness and mood changes.

Decreased sunlight
The average hours of sunlight Utah gets during December is 113. In July, it’s 381. Shorter days during the winter can create delays in the circadian rhythm with respect to the sleep/wake cycle.

More melatonin
When it’s dark out, your body naturally produces more melatonin. Melatonin is a sleep-related hormone and that your body is triggered to produce when it is dark. So, when sun is making shorter appearances your body will produce more melatonin, making you sleepy.

Seasonal depression
Those who suffer from seasonal depression report having less energy, difficulty waking up, decreased motivation and difficulty concentrating.