Differential Psychological Effects of Daytime and Nighttime Heat Exposures on Anxiety: A Randomized Crossover Trial

Abstract

Although heat exposure has been linked to adverse mental health outcomes, little is known about the different psychological effects of daytime and nighttime heat exposure. This study employed a randomized controlled crossover trial design. Between July and August 2025, 41 college students participated in 8 h exposures to daytime heat (32 °C), nighttime heat (30 °C), and a control temperature (26 °C). Anxiety, biomarkers related to inflammation, oxidative stress, neuroendocrine responses, and neuroplasticity were assessed before and after each exposure. Daytime heat exposure was associated with a 24.71% (13.32%–36.11%) increase in anxiety scores and was accompanied by elevated levels of the oxidative stress biomarkers malondialdehyde (MDA) and oxidized low-density lipoprotein (Ox-LDL). Nighttime heat exposure was also associated with an increase in anxiety scores of 27.80% (17.32%–38.29%) and was linked to broader biological alterations, including increased levels of interleukin-1β (IL-1β) and MDA, as well as higher cortisol and lower brain-derived neurotrophic factor (BDNF) levels. Mediation analysis indicated that MDA and cortisol mediated 14.42% and 6.88% of the association between nighttime heat exposure and anxiety, respectively. Heat exposure during the day or at night can adversely affect mental health, with nighttime heat having broader psychological effects and potentially involving distinct biological pathways.