#Body #Sleep #IsThisNormal
The clocks fall back an hour. Your body won’t get the memo.
Maybe you’re wide awake at 5 a.m., or the dark at 5 p.m. makes the evening feel endless. If you’re in treatment or recovering from it, a lost night of sleep can feel like a bigger deal than it used to. Here’s what the research says about the time change and breast cancer, and how to get your sleep back on track, in plain English.
The short answer on the time change and breast cancer
The research doesn’t show that the fall time change raises your risk of breast cancer. A 2024 study of U.S. cancer rates concluded “there is no cancer-risk associated reason to prefer one time over the other.” The change can still throw off your sleep, and sleep problems are already common in treatment. The National Cancer Institute says “as many as half of all people have sleep-related problems during treatment for cancer.”
What the sources say, in brief
- Light runs your clock. Your brain makes melatonin “based on the amount of light the eyes receive,” the NIH says.
- The time change isn’t a cancer risk. Researchers found “no cancer-risk associated reason to prefer one time over the other.”
- Sleep may matter more after diagnosis. A 2025 review found sleep traits mainly linked to “progression in breast cancer patients.”
- Keep the same schedule. “Go to bed and wake up at the same time every day,” the NIH’s heart, lung and blood institute advises.
- Tell your care team. “Talk with your health care team if you have difficulty sleeping, so you can get the help you need,” the NCI says.
What is your body clock, and why does an hour matter?
Your body runs on a roughly 24-hour schedule called a circadian rhythm. The National Institute of General Medical Sciences explains that a cluster of nerve cells in your brain acts as a master clock, and it “controls production of the hormone melatonin based on the amount of light the eyes receive.” Then: “In the evening, a person’s master clock tells their brain to make more melatonin, causing sleepiness.”
When the clocks fall back, the light shifts by an hour against your schedule. It gets dark earlier in the evening and light earlier in the morning, and your master clock has to catch up.
Does the time change affect breast cancer risk?
A large U.S. study says no. The 2024 study in Cancer Research Communications looked at U.S. cancer rates from 2016 to 2020 by where people live within their time zone, a way of comparing conditions like daylight saving time and standard time. The authors concluded that position within a time zone “does not have a significant impact on cancer incidence.”
Breast cancer was one of a few cancers that showed a statistically significant pattern, which the authors call “isolated exceptions” that “may warrant further investigation.” Their bottom line on the debate over daylight saving time: “there is no cancer-risk associated reason to prefer one time over the other.”
What does research say about the body clock and breast cancer?
Scientists have been studying the link for years, mostly in people whose clocks are disrupted night after night. A 2017 review in Neuroscience & Biobehavioral Reviews notes that research on breast cancer risk “has focused largely on adverse effects of night shift work and exposure to light at night.”
A 2024 review in the Journal of Cancer Research and Clinical Oncology describes evidence that “night shift work, jet lag, dietary patterns, and exercise routines” may affect breast cancer risk and progression by disrupting the body’s rhythms. Much of this work is still in the lab. A 2024 review in Frontiers in Immunology looks at how clock genes may affect the immune system’s response to tumors, and a 2022 review in the International Journal of Molecular Sciences reports that “time of treatment” may affect how women with breast cancer respond to radiation. Those are research questions, so don’t change your appointment times on your own. Ask your radiation team if you’re curious.
One older study looked at the seasons directly. In 1991, researchers in New Zealand measured melatonin around the clock in summer and winter in 20 premenopausal women who’d had breast cancer and nine women who hadn’t. Melatonin levels fell in winter among the women who’d had breast cancer. It was a small study, and the authors’ explanation for it remains a theory.
Does sleep matter after a breast cancer diagnosis?
It may, though the research is mixed. A 2025 review of 34 studies in the International Journal of Public Health concluded that “the available evidence points to sleep traits as primarily influencing progression in breast cancer patients and having a relatively small effect on breast cancer incidence.” In that review, poor-quality sleep was linked to a higher risk of the cancer progressing. These studies show links, not cause and effect.
On whether sleep affects the chance of getting breast cancer in the first place, a 2013 study of 110,011 women ages 50 to 79 in the Women’s Health Initiative “does not provide strong support for an association.” A 2017 review of 10 studies in BioMed Research International found that women who slept longer than six or seven hours “might have a significantly increased risk of breast cancer, especially estrogen receptor-positive breast cancer.”
What all of this means for any one woman isn’t clear yet. Sleeping well is still worth protecting. The NCI says a good night’s sleep “may help you to think more clearly, lower your blood pressure, help your appetite, and strengthen your immune system.”
How do you reset your body clock after the time change?
These steps come from the National Heart, Lung, and Blood Institute and the National Cancer Institute.
- Pick a wake-up time and keep it. “Go to bed and wake up at the same time every day.” On weekends, “limit the difference to no more than about an hour.”
- Get outside in daylight. “Spend time outside every day (when possible) and be physically active.”
- Dim the screens at night. Bright light from a TV or computer “may signal the brain that it’s time to be awake.” The NCI suggests you “stop watching television or using other electrical devices a couple of hours before going to bed.”
- Watch the afternoon coffee. “The effects of caffeine can last up to 8 hours.”
- Keep naps short and early. If you have trouble falling asleep at night, “limit naps or take them earlier in the afternoon.” Adults should nap “for no more than 20 minutes.”
- Time your exercise. Stay active during the day, but “exercising a few hours before bedtime may make sleep more difficult.”
- Make the bedroom a cave. “Keep your bedroom quiet, cool, and dark (a dim night light is fine, if needed).”
- Don’t lie there fighting it. “If you do not fall asleep, get out of bed and return to bed when you are sleepy.”
Should you take melatonin?
Not without asking your care team first. Melatonin shows up in the research above as something scientists are studying, not as a recommendation. Ask your oncologist or pharmacist before you start it or any other supplement, especially if you’re in treatment.
When should you talk to your care team about sleep?
If it lasts beyond the time change, say something. The NCI says sleep problems “may be caused by the side effects of treatment, medicines you are taking, long hospital stays, stress, and other factors,” and that “sleep problems that go on for a long time may increase the risk of anxiety or depression.” It lists cognitive behavioral therapy and relaxation therapy as options, and says your doctor may prescribe sleep medicine “for a short period if other strategies don’t work.”
Questions to ask about sleep, from the National Cancer Institute
- Why am I having trouble sleeping?
- What problems should I call you about?
- What steps can I take to sleep better?
- Would you recommend a sleep therapist who could help with the problems I am having?
- Would sleep medicine be advised for me?
Quick question
How has your sleep been since your diagnosis?
Anonymous. We count answers, never names or emails.
More from Babette: Still Tired After Treatment? · Finished Treatment, Still Not Better? · Chemo Brain: Is It Real?
Sources: National Institute of General Medical Sciences, Circadian Rhythms; National Heart, Lung, and Blood Institute, Healthy Sleep Habits, updated March 2022; National Cancer Institute, Sleep Problems in People with Cancer, reviewed August 2021 (“Sleep Problems in People with Cancer” was originally published by the National Cancer Institute); Niu J, Brown C, Law M and others, Longitudinal Position and Cancer Risk in the United States Revisited, Cancer Research Communications, 2024; Samuelsson LB, Bovbjerg DH, Roecklein KA, Hall MH, Sleep and circadian disruption and incident breast cancer risk, Neuroscience & Biobehavioral Reviews, 2017; Circadian rhythms and breast cancer: from molecular level to therapeutic advancements, Journal of Cancer Research and Clinical Oncology, 2024; Circadian rhythms and breast cancer: unraveling the biological clock’s role in tumor microenvironment and ageing, Frontiers in Immunology, 2024; Nelson N, Lombardo J, Matlack L and others, Chronoradiobiology of Breast Cancer, International Journal of Molecular Sciences, 2022; Holdaway IM, Mason BH, Gibbs EE and others, Seasonal changes in serum melatonin in women with previous breast cancer, British Journal of Cancer, 1991; Sleep Traits to the Risk of Breast Cancer Disease Incidence, Adverse Progression and Mortality, International Journal of Public Health, 2025; Vogtmann E, Levitan EB, Hale L and others, Association between Sleep and Breast Cancer Incidence among Postmenopausal Women in the Women’s Health Initiative, Sleep, 2013; Lu C, Sun H, Huang J and others, Long-Term Sleep Duration as a Risk Factor for Breast Cancer, BioMed Research International, 2017. Quotations are from the sources listed. This article is general information and is not a substitute for advice from your care team.
