"most commonly", but there is some evidence that SAD is not triggered only by amount of light:
"This model also explains the otherwise confusing tendency of some SAD sufferers to get depressed in the summer. The problem isn’t amount of light, it’s circadian rhythm disruption – which summer can do just as well as winter can."
"This model also explains the otherwise confusing tendency of some SAD sufferers to get depressed in the summer. The problem isn’t amount of light, it’s circadian rhythm disruption – which summer can do just as well as winter can."
https://slatestarcodex.com/2018/07/10/melatonin-much-more-th...