system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we get sleepy on rainy days?
gpt
Your brain uses certain colors of ambient light to keep track of daytime/nighttime, and to maintain your normal 24 hour rhythm. Most significantly, intense blue light - similar to that of the sky - stimulates the brain to stay awake.

When it's rainy and overcast, you're getting less of that normal daytime light, and your brain interprets this as a sign that it's time to go to bed.