science of us

Why Do I Get Stressed Out at Night?

Photo: Daniel Laflor/Getty Images

I tend to experience the most stress between 6–8 p.m., which is weird, because that’s the one time of day I’m not working or sleeping, and am therefore, at least in theory, “relaxing.” I always assumed this had something to do with the lack of sunlight — resolution and change and growth still seem possible during the day, especially if it’s sunny, but at night? Everything seems closed off and pre-determined, and I always end up thinking about death. Ha.

Anyway, the sunlight thing may or may not be part of it, but apparently evening stress is actually worse than daytime stress, according to a new study published in the journal Neuropsychopharmacology Reports. The authors, who conducted their research at Hokkaido University in Japan, tested how their 27 subjects’ hypothalamic-pituitary-adrenal (or HPA) axes responded to stress incurred at different times of day. The HPA axis connects the central nervous and endocrine systems, and its activation leads to the body’s release of cortisol, the primary human stress hormone. By measuring cortisol levels after a stressful event, the researchers could therefore determine how capably the human body responds to stress in the morning compared to the evening.

After establishing subjects’ baseline cortisol levels, they were asked to prepare and give a presentation to three trained interviewers and a camera, which I am stressed just thinking about. Saliva samples were taken before and immediately after each presentation, and at ten-minute intervals for half an hour afterward. The authors found that students who were given the stress test in the morning had significantly higher cortisol levels in their saliva, while those who were given the evening test experienced no such change. Conversely, there was no significant difference between the post-test heart rates measured in both groups, suggesting that the sympathetic nervous system may be less influenced by time.

I would have thought that a higher stress hormone count meant more stress, but it’s a little counterintuitive, kind of in the same way having a high white blood cell count is actually bad, even though white blood cells are good, because it means something is wrong. If we think of cortisol as one of the body’s strongest tools in addressing stress, its heightened presence is actually (usually) a good thing. Lead author Yujiro Yamanaka explains his study’s findings as follows: in the morning, our bodies have both the HPA axis and our sympathetic nervous system at their disposal, while in the evenings, they have only the latter (in the form of elevated heart rate, for instance). While Yamanaka concedes there is some room for variation due to individuals’ biological clocks (cortisol production also being reliant, in part, on circadian rhythms), generally speaking, he says, his findings suggest “a possible vulnerability to stress in the evening.” To which I say, no kidding!

Why Do I Get Stressed Out at Night?