Importance Light at night causes circadian disruption, which is a known risk factor for adverse cardiovascular outcomes. However, it is not well understood whether personal light exposure patterns predicts and individual’s risk of cardiovascular diseases. Objective To assess whether day and night light exposure predicts incidence of cardiovascular diseases, and whether relationships of light with cardiovascular diseases differ according to genetic susceptibility, sex, and age. Design Prospective cohort study. Setting United Kingdom. Participants N=88,905, age (M±SD) = 62.4±7.8 years, 56.9% female. Exposure Approximately 13 million hours of personal light exposure data, tracked by wrist-worn light sensors (one week each). Main Outcomes and Measures Incidence of coronary artery disease, myocardial infarction, heart failure, atrial fibrillation, and stroke, derived from UK National Health Service records, across a 9.5-year follow-up after light tracking. Results People with the brightest nights (90-100th percentiles) had significantly higher risks of developing coronary artery disease (adjusted-HR range: 1.23-1.32), myocardial infarction (aHRs: 1.42-1.47), heart failure (aHRs: 1.45-1.56), atrial fibrillation (aHRs: 1.28-1.32), and stroke (aHRs: 1.28-1.30), compared to people with dark nights (0-50th percentiles). These relationships were robust after adjusting for established risk factors for cardiovascular health, including physical activity, smoking, alcohol, diet, sleep duration, socioeconomic status, and polygenic risk. Relationships of night light with risk of heart failure and coronary artery disease were stronger for women, and relationships of night light with risk of heart failure and atrial fibrillation were stronger for younger individuals in this cohort. Conclusions and Relevance Night light exposure was a significant risk factor for developing cardiovascular diseases in this cohort. In addition to current preventative measures, avoiding light at night may be a useful strategy for reducing risks of cardiovascular diseases. Question Does personal light exposure predict incidence of cardiovascular diseases? Findings In this study of ∼89,000 adults aged >40 years, exposure to brighter light at night predicted higher incidence of coronary artery disease, myocardial infarction, heart failure, atrial fibrillation, and stroke, independent of established cardiovascular risk factors. Meaning Avoiding exposure to night light may lower risk for cardiovascular diseases. ### Competing Interest Statement Conflicts of Interest Disclosures: M.K.R. has consulted for Eli Lilly and has modest stock ownership in GSK. M.K.R. disclosures are not related to the current work. F.A.J.L.S. served on the Board of Directors for the Sleep Research Society and has received consulting fees from the University of Alabama at Birmingham and Morehouse School of Medicine. F.A.J.L.S. interests were reviewed and managed by Brigham and Women's Hospital and Partners HealthCare under their conflict-of-interest policies. F.A.J.L.S. consultancies are not related to the current work. S.W.C. has consulted for Dyson. A.J.K.P. and S.W.C. are co-founders and co-directors of Circadian Health Innovations PTY LTD. A.J.K.P. and S.W.C. have received research funding from Versalux and Delos. S.W.C. has received research funding from Beacon Lighting. D.P.W, A.C.B., J.M.L., and R.S. have no conflicts of interest to disclose. ### Funding Statement Funding/Support: D.P.W. was supported by the Australian Research Council (DP210102924 and DP220102812). J.M.L. was supported by National Institutes of Health (NIH) grants R35-GM146839 and R01-HG012810. F.A.J.L.S. partly supported by NIH grants R01-HL140574, R01-HL153969, R01-HL164454, and R01-HL167746. Role of the Funder/Sponsor: Funding sources did not play a role in the design and conduct of the study; collection, management, analysis, or interpretation of the data; preparation, review, or approval of the manuscript; or the final decision to submit the manuscript for publication. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This research was conducted using UK Biobank data (Project ID: 6818), and ethical approval was granted by the North West Multi-centre Research Ethics Committee. The Monash University Human Research Ethics Committee of Monash University also gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data used in the present work are available upon application to the UK Biobank team.