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You can't put it on the calendar.

If you experience a mood dip that accompanies the season's shorter days and longer nights, that could be due to a regular body process. "So many people experience a natural slowing of their thyroid in the winter, which allows them to 'hibernate' and conserve energy," says Kelly Brogan, MD, a Manhattan-based board-certified psychiatrist and author of A Mind of Your Own.

What's different about having a thyroid condition like hypothyroidism (or, underactive thyroid) is that the low mood can come on at any time of year—and it doesn't predictably lift once the seasons change. Seeing your doctor may help get to the culprit, which he or she can determine by running a full thyroid panel with a simple blood test, says Aviva Romm, MD, author of The Adrenal Thyroid Revolution.

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You need more than a snooze button.

Not wanting to obey your alarm on those dark mornings? Totally normal here and there. But if you're getting enough sleep and are consistently too tired, physically, to get out of bed and uninterested in tackling the day's tasks ahead, one potential reason could be your thyroid. "When you're lacking thyroid hormone, it slows down your metabolism, which can make you feel depressed and unmotivated," says Romm.

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Your hair is shedding more than usual.

Many seasonal affective disorder (SAD) symptoms, both physical and emotional, can overlap with hypothyroidism symptoms, but it would be extremely rare for someone with a thyroid problem to only have emotional ones, says Brogan. That means thinking 'I've been feeling sad lately' would be just one among a list of issues. "Hypothyroidism isn't just depression, you may also experience other symptoms like constipation, cold hands and feet, hair loss and lackluster skin," Romm says.

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Sunlight doesn't give you a boost.

Post daylight savings, reality sets in: You're commuting to and from work in the dark. And, as you know, that lack of sunlight contributes to SAD. "Light exposure causes a whole host of hormonal and mood changes, and some people are exquisitely sensitive to the decreased light," says Romm.

If you have SAD, light therapy (sitting in front of a window or using a light box) is often recommended and worth a try, certainly; you should start to feel better in one to two weeks, according to research. Otherwise, something else could be going on—and it may be time to make an appointment for a doctor to look at your thyroid more closely.