Can you detect your own seasonal pattern?

Winter makes me miserable.

One of the most common beliefs people hold about their own mood. It may be accurate, or it may be a cultural script attached to a few random bad weeks. The research on seasonal mood is unsettled, and that is the point of this article: whether a given person has a real seasonal pattern is a question that only that person's own data can answer.

The clinical idea

Seasonal mood swings received their clinical name in 1984, when Rosenthal and colleagues described Seasonal Affective Disorder, a recurrent winter depression, and reported that extending the day with bright artificial light appeared to help.¹ The proposed mechanism was straightforward: shorter winter days disrupt mood, and added light restores it. SAD entered the diagnostic manuals, light therapy became a standard recommendation, and the "winter blues" became common knowledge.

The population evidence

Large studies later struggled to confirm it. In 2016, Traffanstedt, Mehta and LoBello analysed depression scores in a U.S. sample of more than 34,000 adults and tested them against season, latitude and sunlight exposure.² Depression was unrelated to all three. The authors concluded that the data did not support seasonal variation as a general feature of major depression, which contradicts the assumption that winter reliably lowers mood across the population.

That leaves a recognised syndrome on one side and a large null result on the other. Both can be true at the same time.

Why the average hides the individual

The largest careful study of weather and daily mood points to the reason. Denissen and colleagues found that the average effect of sunlight and day length on mood was small, while the variation between individuals was substantial, especially in how people responded to daylight.³ Some people are light-sensitive, many are not, and a few feel worse in summer.

Averaged across tens of thousands of people, those opposite responses cancel. A population study reports "no seasonal effect" because the seasonal minority is diluted by everyone who is unaffected, plus the people who respond the other way. For a trait that varies this much between people, the group mean says little about any individual.

Detecting a personal signal

Finding a personal seasonal signal is one of the things daily tracking is suited for, and also one of the harder patterns to establish. Three reasons:

• A season is a slow cycle. It spans a full year, so a single pass through winter provides one observation of the winter effect. As the article on sample size explains, one cycle is far too little for confidence. Confirming a seasonal rhythm as such takes several years.
• Daylight is a better variable than the calendar. Treating daily sunlight, temperature and day length as continuous predictors, logged every day, turns a once-a-year signal into a daily one that is much easier to detect. Weather recorded automatically each day captures these variables.
• Winter is a bundle of confounders. Less daylight arrives together with less exercise, more time indoors, holiday stress, disrupted sleep and lower vitamin D. A simple "December equals low mood" result could reflect any of them, so the daylight signal has to be separated from the rest.

In practice

A population study cannot answer the question for any single person. A year or two of consistent personal data can.