Answer is the low level of happy hormone serotonin in the body cause mood disorder.

When you don't get enough sunlight, your happy hormone serotonin levels drop, which cause a feeling of sadness.

According to studies, sunlight has the ability to produce vitamin D. Vitamin D send the signal to the body to release happy hormone serotonin. Serotonin is known as the "happiness hormone." It improves your mood and keep you out of depression, all of this is researched based.