What causes seasonal affective disorder?

What causes seasonal affective disorder? 
Experts aren’t sure but they believe three factors contribute to SAD.

Not enough sunlight upsets our circadian (biological) clock.  This internal clock affects when we sleep and when we are awake.   We say, ‘the days get shorter’ in fall and winter but that’s not true.    The amount of daylight is decreased during fall and winter.  When our daylight hours shorten, we may not see the sunlight.  Watch a group of people who go to work in the dark and come home in the dark.  After a few days, they will start to feel crabby. (My observation, not a scientific finding.)

Serotonin levels drop because of reduced sunlight.  Serotonin is a neurotransmitter in our brains; a low level of serotonin may contribute to depression.

 

    Melatonin levels may change.  Melatonin is a natural sleep hormone produced by the pineal gland (a small structure inside our heads).  Our retinas (posterior eye involved with vision) transmit light signals to first the hypothalamus and then the pineal gland.  The pineal gland releases melatonin (our natural sleep hormone) when we see low light.  Melatonin level is lowest during the day when the light is bright. Gloomy days may contribute to more melatonin production and sleepiness.