Daylight Saving Time: Does springing forward cause heart attacks?

Every year we spring forward and lose an hour of sleep. But do we also lose a few heart cells? Some headlines claim that heart attacks spike by 24% after daylight saving time begins. In this episode we trace that number back to the research behind it—and what we find is more complicated than the headlines suggest. We examine a famous New England Journal of Medicine letter, a large international meta-analysis, and a massive modern U.S. registry study. Along the way we talk about incidence ratios, relative versus absolute risk, negative controls, and a haunting concept called harvesting. Plus: why bar charts are not for numerical data, why journalists love dramatic numbers, and how a bug collector helped invent daylight saving time.

Statistical topics
• Incidence ratios / incidence rates
• Meta-analysis
• Negative controls
• Relative risk vs absolute risk
• Statistical vs practical significance
• Statistical Sleuthing

Methodological morals
• “A bump in time isn’t always a bump in total.”
• “If you already know the story you want to tell, you can always find a number to tell it.”

References
• Bourke, India. “An obsessed insect hunter: The creepy-crawly origins of daylight saving.” BBC Future, March 31, 2024. bbc.com/future/article/20240308-how-first-sugg…
• Fox-Skelly, Jasmin. “How Daylight Saving Time Affects Your Health.” BBC Future, October 25, 2025. bbc.com/future/article/20251024-how-daylight-s…
• Hurst A, Morfeld P, Lewis P, Erren TC. Daylight Saving Time Transitions and Risk of Heart Attack (pubmed.ncbi.nlm.nih.gov/38888468). Dtsch Arztebl Int. 2024;121(15):490-496. doi:10.3238/arztebl.m2024.0078
• Janszky I, Ljung R. Shifts to and from daylight saving time and incidence of myocardial infarction (pubmed.ncbi.nlm.nih.gov/18971502). N Engl J Med. 2008;359(18):1966-1968. doi:10.1056/NEJMc0807104
• Jiddou MR, Pica M, Boura J, Qu L, Franklin BA.Incidence of myocardial infarction with shifts to and from daylight savings time. (pubmed.ncbi.nlm.nih.gov/23228926) Am J Cardiol. 2013;111(5):631-635. doi:10.1016/j.amjcard.2012.11.010
• Mellour, Richard. “The builder who changed how the world keeps time.” BBC Future, March 11, 2016. bbc.com/future/article/20160310-the-builder-wh…
• Rymer JA, Li S, Chiswell K, et al. Daylight Savings Time and Acute Myocardial Infarction. (pubmed.ncbi.nlm.nih.gov/40924425)JAMA Netw Open. 2025;8(9):e2530442. Published 2025 Sep 2. doi:10.1001/jamanetworkopen.2025.30442
• graph2table.com

Kristin and Regina’s online courses:

Demystifying Data: A Modern Approach to Statistical Understanding (online.stanford.edu/courses/som-xche0033-demys…)
Clinical Trials: Design, Strategy, and Analysis (online.stanford.edu/courses/som-xche0030-clini…)
Medical Statistics Certificate Program (online.stanford.edu/programs/medical-statistic…)
Writing in the Sciences (coursera.org/learn/sciwrite)
Epidemiology and Clinical Research Graduate Certificate Program (online.stanford.edu/programs/epidemiology-and-…)
Programs that we teach in:
Epidemiology and Clinical Research Graduate Certificate Program (online.stanford.edu/programs/epidemiology-and-…)

Find us on:
Kristin - LinkedIn (linkedin.com/in/kristin-sainani-642b5914) &Twitter/X (x.com/KristinSainani)
Regina -LinkedIn (linkedin.com/in/reginanuzzo) &reginanuzzo.com/ReginaNuzzo.com (reginanuzzo.com)

• (00:00) - Intro

• (05:03) - Strange history of daylight saving time

• (16:06) - Swedish NEJM study

• (19:14) - Incidence ratios explained

• (22:13) - What the Swedish study actually found

• (31:11) - Absolute vs relative risk

• (34:27) - Harvesting effect

• (40:10) - 2024 Meta-analysis

• (45:37) - Large modern US study

• (55:23) - Where the “24% increase” came from

• (59:16) - Wrap-up