Anecdotally, I know the amount of light that comes in between the window sill and the curtain can throw off my sleep schedule - I tend to feel most alert around

Anecdotally, I know the amount of light that comes in between the window sill and the curtain can throw off my sleep schedule - I tend to feel most alert around 1am, so I'm a late sleeper, and when the sun rises early (~4:30am in summer around here) I tend to feel unrested or I'm woken after only an hour or two's sleep (and can't get back to sleep even after getting rid of the light). Putting the curtain up on the window sill to block the light completely gets rid of that effect. Oddly artificial light (even with bulbs on the blue-end of the spectrum) tends not to do the same -- if I'm woken for other reasons and use artificial lights and displays then try to get back to sleep, or leave lights on in the room, it's usually not an issue. No idea if that's some sort of dose-response effect or a reaction to the sun cycling rather than being constant (or even placebo potentially).