Is the dark really making me sad? | Ars Technica
the 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. “If someone’s biological clock is running slow and that melatonin rhythm hasn’t fallen, then their clock is telling them to keep on sleeping even though their alarm may be going off and life is demanding that they wake up,” says Kelly Rohan, a professor of psychology at the University of Vermont. Precisely why this should trigger feelings of depression is still unclear. One idea is that this tiredness could then have unhealthy knock-on effects. If you’re having negative thoughts about how tired you are, this could trigger a sad mood, loss of interest in food, and other symptoms that could cascade on top of that.
However, recent insights into how birds and small mammals respond to changes in day length have prompted an alternative explanation. According to Daniel Kripke, an emeritus professor of psychiatry at the University of California, San Diego, 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.
When dawn comes later in the winter, the end of melatonin secretion drifts later, says Kripke. From animal studies, it appears that high melatonin levels just after the time an animal wakes up strongly suppress the making of active thyroid hormone—and lowering thyroid levels in the brain can cause changes in mood, appetite, and energy. For instance, thyroid hormone is known to influence serotonin, a neurotransmitter that regulates mood. Several studies have shown that levels of brain serotonin in humans are at their lowest in the winter and highest in the summer. In 2016, scientists in Canada discovered that people with severe SAD show greater seasonal changes in a protein that terminates the action of serotonin than others with no or less severe symptoms, suggesting that the condition and the neurotransmitter are linked.