Nighttime Heat Increases Anxiety and Alters Body Functions: Scientists Identify Possible Mechanisms

The study was published in the journal Environment and Health. It involved 41 healthy students aged 18–25. The researchers conducted a randomized crossover study in which participants were exposed to both normal temperatures and hot conditions in turn.

Daytime heat exposure lasted from 9 a.m. to 5 p.m. at 32°C, while nighttime exposure took place from 10 p.m. to 6 a.m. at 30°C. A temperature of 26°C served as the control condition. Breaks between experimental sessions allowed participants' bodies to recover.

Before and after each exposure period, the researchers assessed participants' state anxiety, sleep quality and duration, as well as several biological markers associated with inflammation, oxidative stress, hormonal responses and nervous system function.

Both daytime and nighttime heat exposure were associated with increased anxiety. Following daytime exposure, anxiety scores were 24.71% higher than under control conditions, while following nighttime exposure, they were 27.80% higher.

Nighttime heat, however, was associated with a broader range of changes. During the day, participants showed increased levels of malondialdehyde (MDA) and oxidized low-density lipoproteins (Ox-LDL), both markers associated with oxidative stress. At night, in addition to elevated MDA levels, levels of interleukin-1β, which is associated with inflammatory processes, and cortisol increased. At the same time, levels of brain-derived neurotrophic factor (BDNF), which helps maintain nervous system function, decreased.

Participants also slept worse after exposure to nighttime heat: their sleep duration decreased, and sleep quality deteriorated. The researchers suggest that this may partly explain the more pronounced effects of nighttime temperatures. During sleep, the body normally recovers and regulates essential physiological processes, while elevated temperatures make it harder to release heat.

The researchers paid particular attention to cortisol. Under normal conditions, cortisol levels are relatively low at night, so the increase observed after heat exposure may indicate disruptions to normal circadian rhythms and hormonal regulation.

The analysis also suggested that malondialdehyde and cortisol may partly explain the association between nighttime heat and increased anxiety. MDA accounted for 14.42% of this effect, while cortisol accounted for 6.88%.

The researchers note that the findings are particularly relevant in the context of climate change. In many regions, nighttime temperatures are rising faster than daytime temperatures, while periods of extreme heat are becoming more frequent and prolonged.