Nightly Fasting Under 13 Hours Linked to 36% Higher Breast Cancer Recurrence: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Researchers wanted to know whether the length of a woman's nightly fasting window — simply the number of hours between her last meal of the evening and her first meal the next morning — was associated with breast cancer outcomes. While most cancer research focuses on what people eat, this study asked a different question: does when you stop eating matter for cancer prognosis?

The study drew on data from the Women's Healthy Eating and Living (WHEL) Study, which followed women who had been diagnosed with early-stage breast cancer. Nightly fasting duration was calculated from dietary recall data. The primary outcomes measured were breast cancer recurrence and new primary breast cancer. Researchers also examined whether nightly fasting duration was associated with sleep, HbA1c (a measure of blood sugar control over time), and C-reactive protein (a marker of systemic inflammation). For more on how fasting affects metabolic health, see intermittent fasting and insulin sensitivity and intermittent fasting and inflammation.

Who Was Studied

Participant profile: Women diagnosed with early-stage breast cancer (stages I–IIIA), ages approximately 27–74, mean age around 53 years at diagnosis. All participants had completed cancer treatment before entering the study. Diet was assessed via multiple 24-hour dietary recalls collected at baseline and follow-up visits.

How nightly fasting was measured in this study: Researchers calculated the window between the last caloric intake in the evening and the first caloric intake the next morning, using dietary recall data. This was entirely naturally occurring behaviour — participants were not assigned a fasting protocol. The study simply observed how long women naturally fasted overnight and tracked what happened over 7.3 years.

What the Researchers Found

Breast Cancer Recurrence

• The primary finding: women who fasted fewer than 13 hours per night had a 36% higher risk of breast cancer recurrence over 7.3 years of follow-up compared to those who fasted 13 or more hours.
• This association held after adjusting for age, time since diagnosis, tumour characteristics, treatment type, and total daily caloric intake.
• The association was consistent across different patient subgroups.

Blood Sugar Control (HbA1c)

• Short nightly fasting (< 13 hours) was associated with a significantly higher HbA1c of approximately +0.35% (95% CI 0.17–0.53%)
• Higher HbA1c indicates worse blood sugar control over the preceding 2–3 months — a known driver of cancer cell growth pathways including insulin-like growth factor signalling
• This finding suggests one mechanism by which shorter overnight fasting may promote cancer progression: through chronically elevated blood glucose

Sleep Duration

• Women who fasted fewer than 13 hours per night also reported shorter sleep duration on average
• Sleep duration was independently associated with nightly fasting length — suggesting that going to bed later and eating later in the evening are linked behaviours with compounding effects on metabolic and cancer outcomes

What Did Not Change

• No statistically significant association was found between nightly fasting duration and overall mortality during this follow-up period
• No significant difference was detected in C-reactive protein (CRP) levels between fasting duration groups in the primary analysis

What the Researchers Concluded

The researchers concluded that prolonged nightly fasting may be a simple, accessible, and non-pharmacological strategy to reduce the risk of breast cancer recurrence in women who have been diagnosed with early-stage disease. They proposed that longer nightly fasting periods may lower recurrence risk by improving sleep, reducing blood glucose levels, and modulating circadian biology — the body's internal clock system that governs overnight cell repair, hormone secretion, and immune surveillance.

What This Means If You Fast

• 13 hours is a minimum, not a goal. This study identified 13 hours of overnight fasting as the threshold below which recurrence risk rose significantly. Most intermittent fasting protocols — 16:8, 18:6, OMAD — sit well above this threshold by design.
• Blood sugar control may be the mechanism. The HbA1c finding points to chronically elevated overnight blood glucose as a likely driver of the association. Shorter eating windows keep overnight blood glucose lower — see intermittent fasting and insulin sensitivity for the full picture.
• When you eat matters, not just what you eat. This study measured eating behaviour patterns over years and found that the timing of the last meal had measurable consequences for cancer outcomes — independent of total calories consumed.
• Sleep and eating window are intertwined. Eating later shortens the overnight fast and impairs sleep quality simultaneously. For both cancer prevention and general metabolic health, finishing eating earlier in the evening appears to have compounding benefits.
• This is observational evidence, not a trial. The study measured natural behaviour — it did not randomise women to a fasting protocol. The finding is a strong association, not proof of cause and effect. But the strength and consistency of the association across subgroups is notable.
• Fasting during recovery and survivorship may matter long-term. Participants had already completed cancer treatment when their fasting behaviour was measured. This suggests that everyday eating patterns during recovery influence long-term cancer outcomes — not just during active treatment. For broader context on cellular repair during fasting, see what is autophagy and when does it start during fasting.

Study Limitations

• Observational design. Nightly fasting was measured, not assigned. Women who naturally fast longer may differ in other unmeasured health behaviours.
• Dietary recall limitations. Data was collected via 24-hour dietary recalls, which depend on participant memory and may not reflect usual behaviour with perfect accuracy.
• Predominantly female, predominantly white cohort. The WHEL Study population limits generalisability across different demographics and cancer subtypes.
• Mechanism not directly measured. The blood sugar and sleep findings are secondary analyses — the study was not designed or powered to establish mechanism.
• No randomisation. Without a randomised controlled trial, it is not possible to rule out unmeasured confounding factors.
• Relatively small effect of individual fasting hours. The study showed the effect of the binary threshold (< vs ≥ 13 hours) — it was not designed to test whether fasting longer than 13 hours produced additional incremental benefit.

Source

Marinac CR, Nelson SH, Breen CI, Hartman SJ, Natarajan L, Pierce JP, Flatt SW, Sears DD, Patterson RE. Prolonged Nightly Fasting and Breast Cancer Prognosis. JAMA Oncology. 2016;2(8):1049–1055. doi:10.1001/jamaoncol.2016.0164. PMID: 27032582

Frequently Asked Questions

What does this study say about fasting and breast cancer?

Women who fasted fewer than 13 hours per night had a 36% higher risk of breast cancer recurrence over 7.3 years of follow-up compared to women who fasted 13 or more hours overnight. The association was partially explained by worse blood sugar control and shorter sleep duration in the short-fasting group.

Does intermittent fasting prevent breast cancer?

This study cannot make that claim — it looked at recurrence risk in women already diagnosed with early-stage breast cancer, not at whether fasting prevents initial cancer development. The finding suggests that the length of overnight fasting may influence cancer outcomes, but randomised controlled trial evidence is needed before any causal prevention claims can be made.

How long should you fast overnight to reduce cancer risk based on this study?

The threshold identified in this study was 13 hours. Women fasting fewer than 13 hours had significantly higher recurrence risk. The study was not designed to determine whether fasting beyond 13 hours produces additional benefit beyond that threshold.

Why might shorter overnight fasting increase breast cancer recurrence risk?

The researchers propose several biological mechanisms: higher blood glucose (reflected in higher HbA1c) may provide more fuel for residual cancer cells; disrupted sleep impairs overnight immune surveillance and DNA repair; and shorter fasting windows may disrupt circadian rhythms that regulate hormones — including insulin and IGF-1 — both of which have established roles in cancer biology.

Does this study apply to people who have never had breast cancer?

The study was conducted in breast cancer survivors, so direct application to the general population requires caution. However, the biological mechanisms proposed — blood sugar control, sleep quality, circadian alignment — are relevant to general health. Many researchers cite this study when discussing the broader case for longer overnight fasting windows as part of a health-conscious lifestyle.

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