Sleeping well may help to protect men from deadly prostate cancer, a study has found.
Scientists have linked higher levels of the night-time hormone melatonin with a 75pc reduced risk of advanced disease.
Melatonin is produced in the dark at night. It plays a key role in regulating the body's sleep-wake cycle and influences many other functions associated with the body's 24-hour clock, or circadian rhythm.
Low levels of the hormone are typically associated with disrupted sleep.
Scientists studied 928 Icelandic men who were questioned about their sleep patterns and had urine samples tested for levels of a melatonin breakdown product.
Men who reported taking medication for sleep problems, and difficulty falling and staying asleep, had significantly lower amounts of the melatonin marker.
Over a period of seven years, 111 of the men were diagnosed with prostate cancer including 24 with advanced disease. Men whose melatonin marker levels were higher than the middle of the range were 75pc less likely to develop advanced prostate cancer than those with lower values.
Advanced prostate cancer that has started to spread is often aggressive and likely to prove fatal.
"Sleep loss and other factors can influence the amount of melatonin secretion or block it altogether, and health problems associated with low melatonin, disrupted sleep, and/or disruption of the circadian rhythm are broad, including a potential risk factor for cancer," said study leader Sarah Markt, from the Harvard School of Public Health in Boston, US.
"We found that men who had higher levels of melatonin had a 75pc reduced risk for developing advanced prostate cancer compared with men who had lower levels of melatonin.
"Our results require replication, but support the public health implication of the importance of maintaining a stable light-dark and sleep-wake cycle.
"Because melatonin levels are potentially modifiable, further studies of melatonin and prostate cancer risk and progression are warranted."
The research was presented at the American Association for Cancer Research-Prostate Cancer Foundation Conference on Advances in Prostate Cancer Research in San Diego, US.
The study also linked raised melatonin with a 31pc reduced overall risk of prostate cancer, but this finding was non-significant.
"Further prospective studies to investigate the interplay between sleep duration, sleep disturbance, and melatonin levels on risk for prostate cancer are needed," Ms Markt added.