Impact of ambient heat on sleep and mental health

High ambient heat (e.g., temperatures >25C) has negative consequences for mental health. In individuals with pre-existing mental health conditions, high ambient heat is associated with increased symptom presentation, hospitalisations, suicides, and mortality. While less consistent in the literature, even in otherwise healthy individuals, higher ambient temperatures are associated with increased negative mood states, such as sadness, stress, and irritability, which increase the risks of developing mental health disorders. Critically, the mechanisms linking high temperatures to poor mental health remain unknown and rarely studied. Our team is building a program of research to examine three potential mechanisms for heat-induced poor mental health: sleep, cognition, and mood. Multiple reviews show both objectively measured and subjectively reported sleep is disrupted in hotter nocturnal temperatures. Indoor or outdoor ambient temperatures above ~25C reduce sleep duration and quality (hereafter, collectively termed “impaired sleep”), and specifically disrupt REM sleep. Heat-induced sleep impairment has significant and direct implications for mental health. Short sleep and insomnia are well-established, potent, and independent risk factors for development, exacerbation, and relapse in depression and several anxiety disorders, as well as for suicidal behaviours. For example, insomnia more than triples the odds of new onset anxiety and depression (odds-ratios=3.23 and 3.09, respectively). Heat’s impact on REM sleep is particularly concerning for affective disorders, given REM sleep’s role in emotional memory and emotion regulation. Moreover, sleep loss has negative impacts on other factors known to increase the risk or exacerbate the severity of depression and anxiety, including cognition and daily mood states. Gross’ Process Model of emotion regulation has been proposed as an ideal model within which to examine the effects of sleep on affective cognition. Consistent with that framework, our team and others have shown impaired sleep and/or disrupted REM sleep: 1) impairs fear processes, the key mechanism driving most anxiety disorders; 2) impairs emotion regulation and increases reactivity to negative stimuli 3) disrupts cognitive control, which underlies rumination seen in many mood and anxiety disorders; 4) decreases daily positive affect and increases negative affect and anxiety symptoms, and increases daily stress; 5) increases irritability, a transdiagnostic symptom common to many mood and anxiety disorders. Thus, sleep may mediate the relationship between heat and mental health symptoms, specifically depression and anxiety, through both direct and indirect pathways Thesis Possibilities A number of possible thesis topics could be developed to address the broad aims of the program of research. Just two examples are: Qualitative study interviewing members of the community about their perceived relationships among nocturnal heat, sleep, cognition, mood, and mental health, as well as mitigation strategies they currently employ to cope with heat at night. Lived experience input is critical to ensure we understand the inputs and outcomes important to those living in chronically hot environments and/or experience heat-induced exacerbations of mental health symptoms. Including older adults and others known to be especially vulnerable to the negative effects of ambient heat. EMA study recording sleep and heat stress in participants’ bedrooms nightly for a period of multiple weeks. Such a study would measure heat and humidity in the participant bedroom, objective and subjective sleep, heat mitigation strategies employed by participants, and daily reports of mood and cognition.