Seasonal variations in day and night length affect mood and metabolism.

“So as all you know, the Earth spins once every 24 hours on its axis. So part of that day, we're bathed in sunlight, depending on where we are. The other half of the day or part of the day, we're in darkness. The Earth also travels around the sun. 365 days is the time that it takes one year to travel around that sun. The Earth is tilted. It's not perfectly upright. So the Earth is tilted on its axis. So depending on where we are in that 365-day journey and depending on where we are in terms of hemisphere, Northern Hemisphere or Southern Hemisphere, some days of the year are longer than others. Some are very short. Some are very long.”