system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it seem that people always have more sad thoughts/are in their feelings in the nighttime compared to the day?
gpt
There's a hormone your body produces called cortisol.  This does a LOT of things, but importantly when there's plenty of it it makes you feel like you're more energetic which often corresponds with mood.  Your cortisol levels change throughout the day, with their highest levels in the late morning and their lowest levels at midnight/in the early hours of the morning.  This is also a very noticeable thing in people with depression - they often feel significantly worse in the mornings.  This won't be the only reason, but it probably is one.