Low levels of melatonin, the hormone that regulates sleep, more than doubles the risk of diabetes in women, a study has found.
Researchers made the connection after analysing data on 740 women with and without the disease.
Melatonin, a popular remedy for jet lag, is best known for its role in helping to control the body's sleep-wake cycle. But it also has many other biological functions, including some linked to the blood sugar hormone insulin.
None of the women taking part in the study had diabetes at the start. Over a 12-year period, researchers identified 370 who developed Type-2 diabetes and compared them with a matched 370 who did not.
Melatonin secretion varied widely among the participants.
Women who produced the least were more than twice as likely to develop diabetes as those who secreted the most.
Study leader Dr Ciaran McMullan, from Brigham and Women's Hospital in Boston, US, said: "This is the first time that an independent association has been established between nocturnal melatonin secretion and Type-2 diabetes risk.
"Hopefully this study will prompt future research to examine what influences a person's melatonin secretion and what is melatonin's role in altering a person's glucose metabolism and risk of diabetes."
The research, forming part of the US Nurses' Health Study, is reported in the Journal of the American Medical Association.
"It is interesting to postulate from these data, in combination with prior literature, whether there is a causal role for reduced melatonin secretion in diabetes risk," wrote the authors.
Whether or not the same association is seen in men remains to be investigated.