I believe it, because my sleep habits and general demeanor in the summer versus the winter are vastly different.
I have a sun lamp (I call it my SAD boi lamp) I use during the winter months in the morning and I absolutely notice the benefits of it right away. I feel more energized and in a much better mood. I remember reading somewhere (maybe from the Philips product website) that as little as 15 minutes of sun lamp time is enough to be beneficial, but I leave mine on for about an hour.
The circadian rhythms benefits are absolutely true too. Your body says ok, the sun just rose, so let me set my awake timer countdown now. I would highly recommend it even if you don’t have seasonal affective disorder for the sleep improvement alone. I find it’s much easier to go to bed and after a few days of use, it’s easier to get out of bed as well!