Long article, but here is the main part: > Leading theory is the ‘phase-shift hypothesis’: the idea that shortened days cause the timing of our circadian rhyth

Long article, but here is the main part: > Leading theory is the ‘phase-shift hypothesis’: the idea that shortened days cause the timing of our circadian rhythms to fall out of sync with the actual time of day, because of a delay in the release of melatonin. Levels of this hormone usually rise at night in response to darkness, helping us to feel sleepy, and are suppressed by the bright light of morning. Edit: And alternative hypothesis from the article: > When melatonin strikes a region of the brain called the hypothalamus, this alters the synthesis of another hormone—active thyroid hormone—that regulates all sorts of behaviours and bodily processes.