Background Environmental heat exposure can negatively impact mental health. Evidence for its effect on mood disorder symptoms is inconsistent. Current studies are limited by poor temporal and geographical resolution. Methods We used ecological momentary assessment (EMA) data from the smartphone app juli to investigate the association between real-time mean and maximum ambient temperature collected from smartphone geolocation, and depressive and manic symptom scales, every two weeks, in adults with depression and bipolar disorder. We used negative binomial mixed-effects regression models, controlled for demographic and weather variables, and stratified by season. Results We analysed data from 4,000 participants with depressive symptom scores and 2,132 with manic symptom scores, between 2021 and 2023. We found that each 1°C increase in mean daily temperature in the preceding two weeks was associated with a 0.2% reduction in depressive symptom scores (coeff 0.998, 95%CI 0.997-0.999) and a 0.4% increase in manic symptom scores (coeff 1.004, 95%CI 1.001-1.007). Associations between maximum temperature and symptom scores followed a similar pattern. Limitations We were unable to capture several socio-demographic covariates, had limited geographical information due to privacy regulations, and included a non-random sample. Conclusions We found evidence that higher temperatures were associated with increased manic symptoms and decreased depressive symptoms, indicating an important relationship between temperature and the mood disorder continuum. With global heating, there is a need to understand the impact of temperature on mood symptoms, to provide targeted clinical prevention and support. This study demonstrates potential for EMA methods to inform our understanding of these links.

JFH and AK have received consultancy fees from the Wellcome Trust and juli Health. JFH is a co-founder of juli Health. AK is employed by Zinc VC.

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PC is supported by the National Institute for Health Research Academic Clinical Fellowship. JD, DPJO, and JFH are supported by the National Institute for Health Research, University College London Hospital, Biomedical Research Centre. JFH and DPJO are supported by NIHR North Thames Applied Research Collaboration. JD is supported by the National Institute of Health Research (grant number NIHR302266). JFH is supported by the UK Research and Innovation grant MR/V023373/1. JFH and AK have received consultancy fees from the Wellcome Trust and juli Health. JFH is a co-founder of juli Health. AK is employed by Zinc VC. NL is supported by UK Research and Innovation grants MR/V023373/1 and MR/W014386/1, the University College London Hospitals NIHR Biomedical Research Centre, and the NIHR North Thames Applied Research Collaboration. RT is supported by the National Institute for Health and Care Research (NIHR) School for Public Health Research (SPHR) (Grant Reference Number NIHR 204000) The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care. ELL is a grateful recipient of financial support from Lenore England.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Research Ethics committee of University College London gave ethical approval for this work (ID 19413/002).

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An anonymised dataset is available from juli Health, given appropriate permissions.