Depressive symptoms may look similar, but there are various underlying causes.

While depressive symptoms like fatigue, low mood or low energy can look similar in many people, the complex condition can be caused by a combination of factors, leading to different subtypes of depression.

Recent research identified a new subtype that may be driven by the body’s internal clock, disrupting sleep and creating a perpetual state of jet lag.

Since depression is caused by several factors, researchers have been looking into biological issues that could be causing symptoms, including the circadian rhythm or clock.

The circadian clock is the body’s internal 24-hour rhythm, managing hormones, digestion, mood and sleep-wake cycles.

“When it’s off, you feel jet-lagged all the time,” lead researcher Ian Hickie told ScienceAlert. “You feel tired, you feel exhausted, you gain weight, you can’t concentrate, you sleep poorly and your behavior changes.”

The study, published in the British Journal of Psychiatry, found that one in four people with depression could have “circadian depression” caused by a dysregulated body clock.

Looking at data from 11,380 participants in the ongoing Australian Genetics of Depression Study, the researchers found a quarter of participants met at least three of six criteria that defined the circadian depression subtype.

One is that symptoms will change with the seasons, due to the length of days and natural light shortening.

The second criterion is that the midpoint of sleep (the exact time halfway between the moment you fall asleep and the moment you wake up) is delayed compared to the general adult population, roughly between 2 and 4 a.m.

Those with circadian depression also experience social jet lag, with their sleep midpoint differing by several hours on workdays than on non-working days. This means their sleep patterns don’t match the typical timing that society is designed around.

A fourth criteria point is having an evening chronotype, which previous research has suggested may lead to more depressive symptoms due to increased bad decisions and impulsive behavior the later someone stays up.

The last two criteria found that the participants would sleep longer than the average person, if given the chance, and reported feeling very tired in the morning.

The research also found that participants with circadian depression experienced more side effects from antidepressant medications and found the treatments less effective.

“Depression is not a uniform illness and understanding the characteristics of different subtypes mean that we can give people earlier access to treatments that will be most effective for them,” first study author Emiliana Tonini told ScienceAlert.

The study found those with the circadian subtype were more likely to experience weight gain during depressive episodes, had a higher risk for inflammatory and cardiovascular health problems and neurodevelopmental disorders, like ADHD.

They also experienced depression earlier and reported more severe symptoms, such as mania, psychosis and suicidality.

Since typical medications are less effective and this subtype is linked to the body’s internal clock, researchers suggest treatments should target the circadian rhythm instead, using light therapy and melatonin.