Reports of winter low mood rise with distance north across the United States. The gradient is one of the more informative features of the phenomenon.
Day length changes far more at higher latitudes
The difference between midsummer and midwinter daylight is modest in the southern states and substantial in the northern tier.
Because winter mornings there stay dark well past the time people wake for work and school, the light signal arrives late or not at all during the commute.
The pattern of reports follows this gradient closely enough that light exposure, rather than temperature or snow, is the leading candidate explanation.
Morning light sets the clock
The body's daily timing system is entrained largely by light reaching the eye, with morning exposure having the strongest effect on where the cycle sits.
Weak or late morning light allows the internal cycle to drift later, so waking occurs at a point the body still treats as night.
That mismatch between clock time and internal time produces grogginess, difficulty rising and low daytime energy that persists across the season.
Indoor light is dimmer than it seems
Vision adapts, so a lit room feels adequately bright while delivering a small fraction of the light intensity available outdoors even on an overcast day.
A person who commutes in darkness, works under artificial light and returns after sunset may receive very little of the signal the system relies on.
This is why stepping outside briefly during daylight is repeatedly suggested, and why it is not the same as turning on more lamps.
The pattern is recognized clinically
Recurrent depressive episodes with a seasonal pattern are a recognized presentation, distinct in timing from ordinary dislike of dark evenings.
Treatments studied for it include timed light exposure, structured psychotherapy and medication, and the choice depends on the individual and their history.
Light devices are sold directly to consumers, but timing and intensity matter and there are conditions under which they are inadvisable, so the decision belongs with a clinician.
Where the explanation stops
Latitude does not account for everything, since people at the same latitude differ substantially and some report the opposite seasonal pattern.
Work schedules, indoor occupations, existing sleep timing and individual sensitivity all modify the exposure a person actually receives.
Persistent low mood in any season, seasonal or not, is a reason to speak with a doctor rather than to wait for the calendar to change it.