Working the night shift might lead to hormonal and metabolic changes that raise risks for obesity, diabetes and heart disease, researchers at Brigham and Women's Hospital and Harvard Medical School in Boston have found.

The researchers, led by Frank Sheer, an instructor of medicine in the division of sleep medicine, have found that night shift work affects much of the human body's biological clock - its circadian rhythm. The internal biological clock keeps day-shift time even when a person goes on the night shift, the team explains.

"In the long run, the physiological impact of shift work on several markers involved in the regulation of body weight - leptin, insulin, cortisol - seems to contribute to the increased risk for the development of diabetes, cardiovascular disease and obesity," said the researchers.

Scheer and his team have reported the findings in the 2 March online issue of the Proceedings of the National Academy of Sciences.

The authors point out that about 8.6 million Americans perform shift work, which the National Sleep Foundation defines as any type of schedule that falls outside the standard nine-to-five norm for business hours. In the United States, factory workers, hospital staff, policemen, firefighters, pilots, road crews and truck drivers are some of the positions that commonly entail some degree of shift work.

This type of work has been previously associated with gastrointestinal problems, fatigue and poor sleep, the researchers noted. Such complications are thought to arise from a chronic disconnect between the waking and eating habits the work demands and the body's innate 24-hour sleep/wake clock, or circadian rhythm.

To explore how such a misalignment might raise the risk for developing serious health issues, Scheer and his colleagues conducted a laboratory test designed to mimic the acute effects of jet lag and/or the chronic impact of regular shift work. In the experiment, the bodily responses of five men and five women were tracked as they stuck to an ever-changing sleep/eat schedule for 10 days.

By the study's conclusion, all the volunteers had eaten and slept across all phases of the circadian cycle, as they followed a daily schedule artificially fashioned along 28-hour blocks.

The results: circadian misalignment provoked a drop in levels of the weight-regulating hormone leptin. Plummeting leptin levels could hasten the onset of obesity and heart disease by prompting increases in appetite and decreases in activity, the researchers said.

Furthermore, changes in blood sugar levels and insulin levels also occurred, resulting in impaired glucose tolerance and decreased insulin sensitivity.

In particular, three participants with no prior history of diabetes developed glucose levels that resembled those of pre-diabetic people after eating on the misaligned schedule. Daytime blood pressure levels were also found to be elevated among these volunteers.

The degree of hormonal change was highest when participant schedules were set 12 hours off the normal sleep/wake cycle - that is, when participants were asked to sleep throughout the day and stay awake through the night.

The researchers found that when they shifted volunteers' sleep-wake cycles to resemble those of night-shift workers, the participants experienced changes in their stress and hormones that control appetite. During this phase - when they were sleeping during the day and up at night - the volunteers' bodies got seriously out of rhythm.

Yet despite the strength of the findings, Scheer cautioned that more research is needed before drawing too many conclusions.

"First of all, this is an in-laboratory study of short duration," he observed. "So we don't yet know if circadian misalignment has a similar impact in the long run in a real-life setting where people are performing night shift work. We also need to look at how different people might respond differently." 

Scheer noted that "Because shift work typically affects people's alertness levels, and GI functioning, and those who don't cope well with this are likely to drop out. This implies that means that those who continue with this kind of work might not be so susceptible to such problems, and may be less sensitive to this kind of misalignment. These are all questions for the future."

Scheer added, ''These findings do not apply only to those performing shift work, but may also have implications for people suffering from circadian rhythm sleep disorders, including advanced and delayed sleep phase syndrome and many blind people experiencing circadian misalignment due to the absence of resetting their body clock by light.

''In addition, because these changes were observed within just a few days of misalignment, circadian misalignment may even temporarily affect millions of international travellers each year.''