Sleeping too little or too much has not been reliably linked to cancer on its own. A large 2018 review of studies covering more than 1.5 million people found no overall link between sleep duration and cancer risk. The clearer signal is long-term night shift work, which the World Health Organization cancer agency (IARC) classifies as probably carcinogenic, based on limited evidence in humans. For people who already have cancer, sleep problems are very common and treatable.
Work nights or rotating shifts? The shift work sleep calculator plans when to sleep, nap and avoid light around your shifts.
Does Lack Of Sleep Cause Cancer?
The evidence does not show that. A systematic review and meta-analysis pooled 65 studies with 1,550,524 participants and 86,201 cancer cases. Neither short nor long sleep was associated with higher cancer risk overall (Chen et al., BMC Cancer 2018):
| Comparison | Result | What it means |
|---|---|---|
| Short sleep, all cancers | Odds ratio 1.01 | No meaningful difference |
| Long sleep, all cancers | Odds ratio 1.02 | No meaningful difference |
| Short sleep, Asian populations | Odds ratio 1.36 | Possible link in a subgroup |
| Long sleep, colorectal cancer | Odds ratio 1.21 | Possible link for one cancer |
The two subgroup signals were not confirmed when the researchers analysed sleep as a continuous dose, so the authors called for further research rather than drawing firm conclusions. An odds ratio of 1.00 would mean no difference at all.
So if you have had a few bad weeks of sleep, there is no reason to fear that it has caused cancer. Sleep still matters for your health in other ways, covered below.
Why Is Night Shift Work Called A Probable Carcinogen?
In 2019 an IARC working group classified night shift work as probably carcinogenic to humans (Group 2A). The decision rested on limited evidence of cancer in humans for cancers of the breast, prostate, colon and rectum, sufficient evidence in experimental animals, and strong mechanistic evidence in experimental animals (IARC).
IARC defined night shift work as work during the usual sleeping hours of the general population, including long-haul air travel across time zones, and noted that disruption of the body clock (circadian rhythm) is its most marked effect (IARC). That points to circadian disruption, rather than sleep length alone, as the main area of concern.
What Does Probably Carcinogenic Mean?
IARC groups describe how strong the evidence is that something can cause cancer, not how large the risk is. Group 2A sits below Group 1 (carcinogenic). “Limited evidence in humans” means studies have found a link, but the evidence is not strong enough to be sure it is causal. For an individual night worker, the classification is a reason to look after sleep and general health, not a reason for alarm.
| Claim you may have read | What the evidence says |
|---|---|
| “Sleeping under 7 hours raises cancer risk by 30%” | Not supported; the large 2018 meta-analysis found no overall link |
| “Night shifts cause cancer” | Probable, not proven; limited evidence in humans |
| “Poor sleep quality raises cancer risk by 40%” | No reliable source for this figure |
What Can Night Shift Workers Do?
No study has shown that a particular sleep routine lowers cancer risk in shift workers, but good habits protect sleep and overall health:
- Protect one main sleep block after each night shift in a dark, quiet, cool room.
- Keep a regular pattern where your rota allows, rather than switching between day and night sleep often.
- Use light on purpose: bright light during the shift, sunglasses on the way home.
- Keep up with recommended cancer screening and general health checks.
- Do not smoke, limit alcohol, stay active and keep a healthy weight; these are established ways to lower cancer risk.
Our full plan is in how to sleep after a night shift.
How Does Cancer Affect Sleep?
Insomnia, trouble falling or staying asleep, is common in people being treated for cancer. Studies show that as many as half of all people have sleep-related problems during cancer treatment (National Cancer Institute). Causes include side effects of treatment, medicines, long hospital stays and stress.
| Cause | Examples |
|---|---|
| The cancer itself | Pain, breathing problems, itching, fever |
| Treatment | Side effects of chemotherapy, hormone therapy and other medicines |
| Hospital stays | Noise, light and night-time checks |
| Stress | Worry after a diagnosis, anxiety, low mood |
The sleep disorders most likely to affect people with cancer are insomnia and an abnormal sleep-wake cycle (NCI PDQ).
Why Does Sleep Matter During Cancer Treatment?
Poor sleep can make it hard to stay alert and involved during the day, remember treatment instructions and make decisions. Being well rested can improve energy and help people cope with the side effects of cancer and treatment, and sleep problems that go on for a long time may increase the risk of anxiety or depression (NCI PDQ).
What Helps People With Cancer Sleep Better?
The National Cancer Institute describes these approaches (NCI PDQ):
- Treating the cause, such as pain, nausea or hot flashes, through supportive care.
- Cognitive behavioral therapy, which reduces anxiety about sleep and replaces negative thoughts about sleep with more helpful ones.
- Good sleep habits, such as a regular routine and a comfortable, quiet bedroom.
- Sleep medicines for a short time if treatment without drugs does not help.
Telling your care team about sleep problems matters, because they are part of your care, not a side issue. Our guide to beating insomnia explains the steps behind CBT for insomnia.
Does Sleep Still Matter For Long-Term Health?
Yes. Even though short sleep has not been reliably linked to cancer, ongoing sleep deficiency can raise your risk of some chronic health problems and affects how well you think, react, work and learn (NHLBI). Getting enough regular sleep is worth doing for its own sake. The sleep debt calculator shows how much sleep you are missing each week.
When Should You See A Doctor?
See a doctor about any new lump, unexplained weight loss, bleeding, a persistent cough or other symptoms that worry you, whatever your sleep is like. If you have cancer, tell your care team about sleep problems that last more than a few weeks, or about pain, breathlessness or low mood that keeps you awake.
Plan safer sleep around night shifts with the shift work sleep calculator.
Sleep And Cancer FAQs
Can Lack Of Sleep Cause Cancer?
There is no reliable evidence that short sleep alone causes cancer. A large meta-analysis found no overall link between sleep duration and cancer risk.
Is Night Shift Work Linked To Cancer?
IARC classifies night shift work as probably carcinogenic, based on limited evidence in humans for breast, prostate, colon and rectal cancer.
Should I Stop Working Night Shifts?
Not necessarily. Discuss your health and work pattern with a doctor, protect your sleep, and keep up with recommended screening.
Why Can I Not Sleep During Cancer Treatment?
Pain, medicines, hospital stays and stress are common causes. As many as half of people have sleep problems during treatment.
Are Sleeping Pills Safe During Cancer Treatment?
Sometimes, for a short time. Doctors usually try non-drug approaches first and choose medicines that suit your treatment.
Does Sleeping A Lot Raise Cancer Risk?
Long sleep was not linked to overall cancer risk. One subgroup result for colorectal cancer was not confirmed in the dose analysis.