Controlled heat exposure raised short-term anxiety in young adults, while nighttime heat disrupted sleep and affected a wider range of biological markers linked to stress and brain health.
Study: Differential Psychological Effects of Daytime and Nighttime Heat Exposures on Anxiety: A Randomized Crossover Trial. Image credit: Igor Barilo/Shuttertock.com
A new study published in the journal Environment and Health reveals that both daytime and nighttime heat exposure increased state anxiety levels among college students, with nighttime exposure associated with a broader range of biological alterations.
Why nighttime heat may pose health risks
Global warming due to climate change has significantly increased the frequency, duration, and intensity of extreme heat waves worldwide. This is concerning because heat is a major environmental risk factor that substantially affects both physical and mental health.
One of the most prominent features of climate change is that nighttime temperatures are rising more rapidly than daytime temperatures in many regions. Nighttime heat exposure reduces the body’s ability to release heat, which can disrupt normal sleep cycles and recovery processes and exert effects on human health.
Despite observational evidence linking heat exposure to adverse health effects, experimental evidence on potential biological processes driving this association is largely lacking.
To address this gap in the literature, researchers conducted a randomized controlled crossover trial to explore the psychological effects of daytime and nighttime heat exposure and to identify potential mechanisms driving these effects.
Recreating daytime heat and hot sleeping conditions
The trial was conducted on 41 healthy college students aged 18 to 25 years. They were randomly assigned to the order in which they experienced heat exposure and a thermoneutral control condition (26°C).
Participants in the first group underwent heat exposure first, followed by a washout period of more than 3 days before undergoing the thermoneutral condition. Participants in the second group followed the reverse order.
The daytime heat exposure (32°C) and nighttime heat exposure (30°C) were treated as two independent study phases separated by a period of at least one week. Daytime exposures lasted eight hours from 9 a.m. to 5 p.m., while nighttime exposures ran from 10 p.m. to 6 a.m. to simulate typical sleeping hours.
Participants’ state anxiety levels and biomarkers related to inflammation, oxidative stress, cortisol, and brain-derived neurotrophic factor (BDNF) were assessed before and after the exposure.
Anxiety rises with both daytime and nighttime heat
The overall study findings indicated that heat exposure significantly increased state anxiety scores and was associated with changes in multiple biological indicators. Specifically, heat exposure was associated with increased inflammatory and oxidative stress biomarkers, increased cortisol levels, and reduced BDNF levels.
The analysis stratified by exposure time revealed that daytime and nighttime heat exposure were associated with 24.71% and 27.80% higher state anxiety scores, respectively.
Notably, nighttime heat exposure was associated with a broader range of significant biological changes than daytime heat exposure. Specifically, daytime heat exposure was associated with elevated levels of the oxidative stress biomarkers malondialdehyde (MDA) and oxidized low-density lipoprotein (Ox-LDL), whereas nighttime exposure was associated with elevated levels of the inflammatory biomarker interleukin-1β (IL-1β) and MDA, together with increased levels of cortisol and reduced levels of BDNF.
Furthermore, nighttime exposure was associated with poorer sleep quality and shorter sleep duration.
Regarding potential mediators of the effect of heat exposure on participants’ anxiety levels, the analysis indicated that MDA and cortisol mediated 14.42% and 6.88% of the association between nighttime heat exposure and anxiety, respectively. However, none of the tested biomarkers showed any significant mediating effect for daytime heat exposure.
Nighttime heat reveals distinct pathways linked to anxiety
This randomized controlled crossover trial reveals that heat exposure during the day or night can significantly increase state anxiety scores and alter the levels of inflammatory, oxidative stress, and neuroendocrine biomarkers in healthy college students. However, the trial finds significant biological mediation only for nighttime heat exposure, involving oxidative stress and neuroendocrine pathways.
All trial experiments were conducted under strictly controlled environmental conditions, which allowed researchers to effectively assess the short-term effects of heat exposure on state anxiety. Moreover, by incorporating both daytime and nighttime heat exposure conditions and combining them with multidimensional biological indicators, the trial provides direct experimental evidence that acute heat exposure can affect state anxiety under controlled conditions, while offering clues to the biological pathways that may contribute to these effects.
The observed broader biological effect of nighttime heat exposure may be due to the impairment of the body’s recovery and maintenance processes during sleep. Given that good-quality sleep is vital for psychological, neurological, and cognitive regulation, the observed disruption of sleep quality and duration may potentially explain the adverse health effects of nighttime heat exposure.
Cortisol is a crucial neuroendocrine hormone and a key marker of hypothalamic-pituitary-adrenal (HPA) axis activity, which plays a vital role in the body’s response to stressors. Under normal physiological conditions, cortisol levels remain relatively low at night. In this trial, an increase in cortisol levels was observed in response to nighttime heat exposure, suggesting that nighttime heat may disrupt normal circadian rhythms and neuroendocrine homeostasis.
Taken together, these findings suggest that oxidative stress and neuroendocrine responses may partly contribute to the effect of nighttime heat exposure on anxiety, with MDA and cortisol emerging as significant mediators.
Given the global increase in nighttime heat events, these findings suggest that future heat-related health management and climate adaptation strategies should focus more on the potential physical and mental health impacts of nighttime heat exposure.
The trial enrolled only young and healthy college students, which may restrict the generalizability of its findings. The study also assessed only short-term, acute heat exposure. Future research is needed to evaluate the effect of long-term or repeated heat exposure on mental health.
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