Shift workers getting too little sleep at the wrong time of day may be increasing their risk of diabetes and obesity, according to researchers.
The team is calling for more measures to reduce the impact of shift working following the results of its study.
Researchers controlled the lives of 21 people, including meal and bedtimes.
The results, published in Science Translational Medicine, showed changes to normal sleep meant the body struggled to control sugar levels.
Some participants even developed early symptoms of diabetes within weeks.
Shift work has been associated with a host of health problems.
Doctors at Brigham and Women's Hospital, in the US, were trying to study its effects in a controlled environment.
Lower insulin levels
The 21 health-trial participants started with 10 hours' sleep at night. This was followed by three weeks of disruption to their sleep and body clocks.
The length of the day was extended to 28 hours, creating an effect similar to a full-time flyer constantly getting jet lag.
Participants were allowed only 6.5 hours' sleep in the new 28-hour day, equivalent to 5.6 hours in a normal day. They also lived in dim light to prevent normal light resetting the body clock.
During this part of the study, sugar levels in the blood were "significantly increased" immediately after a meal and during "fasting" parts of the day.
The researchers showed that lower levels of insulin - the hormone that normally controls blood sugar - were produced.
Three of the participants had sugar levels which stayed so high after their meals they were classified as "pre-diabetic".
They also highlighted a risk of putting on weight as the body slowed down.
"The 8% drop in resting metabolic rate that we measured in our participants... translates into a 12.5-pound increase in weight over a single year," they wrote.
Lead researcher Dr Orfeu Buxton said: "We think these results support the findings from studies showing that, in people with a pre-diabetic condition, shift workers who stay awake at night are much more likely to progress to full-on diabetes than day workers.
"Since night workers often have a hard time sleeping during the day, they can face both circadian [body clock] disruption working at night and insufficient sleep during the day.
"The evidence is clear that getting enough sleep is important for health, and that sleep should be at night for best effect."
The research group called for more efforts to reduce the health impact of shift working.
Dr Matthew Hobbs, head of research at Diabetes UK, said: "This is an interesting study which shows that under extreme conditions involving sleep deprivation and 'tricking' the body clock, participants produced less insulin and therefore had higher blood glucose levels then when they were able to sleep normally and live according to normal daily rhythms."
He cautioned that the laboratory conditions were not the same as working nights.
"Clearly, this does not equate to the normal experience of shift workers who are able, for example, to use bright lights when not sleeping.
"The study also involved only 21 people. For these reasons, it is not possible to conclude that the findings would translate to real conditions in the wider public."