A person with depression might sleep much longer than usual and still wake feeling exhausted. Later that evening, when they want to go to bed, they may find themselves wide awake. Researchers are investigating whether patterns like these could help explain why antidepressants work better for some people than others.

The clue may lie in the body’s internal clock, which helps regulate when someone sleeps, wakes and feels alert.

An Australian study examined 11,380 people, predominantly with severe or recurring depression. About 23% met the researchers’ criteria for a proposed subtype called “circadian depression”, named after the body’s roughly 24-hour internal clock.

Participants needed at least three of six features, including being a night owl, sleeping longer during depression, morning tiredness and seasonal changes in mood or behaviour. The other features involved habitually sleeping and waking later, and having different sleep schedules on working days and days off.

Compared with other participants, this group reported less benefit and more side-effects from two common types of antidepressants, SSRIs and SNRIs, which affect how brain cells communicate through chemical messengers. However, some people with these sleep patterns still reported benefiting from these medicines.

The researchers also found genetic associations with later sleep timing, reduced sensitivity to insulin, the hormone that helps regulate blood sugar, and a protein involved in inflammation, the immune system’s response to injury or infection. These associations reflect inherited tendencies, rather than evidence that participants........

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