It is well known that workers on the night shift disturb their sleep-wake cycles and so, too, their circadian rhythms — the 24-hour cascade of physical, mental, and behavioral changes occuring in all humans, in all organisms. This disruption causes shift workers to be more accident prone and even harms their long-term health. The subtle influences of circadian rhythms also may be affecting the rest of us.

A new study finds time of day and sleep deprivation have a significant effect on our metabolism, suggesting there is a "best time" to test for diseases, such as cancer and heart disease, as well as for administering medicines. “Our results show that if we want to develop a diagnostic test for a disease, it is imperative to take the time of day when taking blood samples into account, since this has a significant effect on metabolism,” said Dr. Debra Skene, lead author and professor at the University of Surrey. “This is also key for administering medicines and determining when they will be at their most effective.”

The Clock Within Your Brain

Within our brains is a “master clock” that controls circadian rhythms, and it consists of a group of nerve cells called the suprachiasmatic nucleus, or SCN. The SCN is located in an area of the brain just above where the optic nerves from the eyes cross. Although produced naturally within our bodies, our circadian rhythms respond primarily to light cues in our environment and so follow a roughly 24-hour cycle. Circadian rhythms influence our body temperature, when certain hormones are released, other bodily functions, and when we sleep. For this reason, circadian rhythms are linked to insomnia, which is the most prevalent sleep disorder among adults and affects about 30 percent of all adults.

To better understand the link between body clock disruption and sleep deprivation, then, a team of researchers designed an experiment to understand how much time of day corresponds to variations in metabolism. Enlisting the help of 12 healthy male volunteers, the researchers requested the participants stay in an environment controlling sleep, light, meals, and even posture. Then the researchers collected blood samples every two hours to see how metabolic biomarkers change during the day. For the first 24 hours, the researchers subjected participants to a normal wake/sleep cycle in the controlled environment. Yet, for the next 24-hour period, the researchers subjected the participants to constant wakefulness, as a way to investigate the effects of sleep deprivation on their metabolic rhythms.

And, indeed, the researchers discovered differences in metabolic processes based on how much participants were allowed to sleep. In fact, 27 metabolites, including serotonin, significantly increased during sleep deprivation compared to periods of a normal wake/sleep cycle. "Clear daily rhythms were observed in most metabolites, with 24 h wakefulness mainly reducing the amplitude of these rhythms," wrote the authors in their study.

The researchers believe their work will be helpful to pharmacologists. “Our findings are likely to be important in interpreting the results of blood tests, and in understanding why some individuals respond differently to medication," said Dr. Florence Raynaud, a group leader at The Institute of Cancer Research. "They also set reference points for future studies looking at the connection between metabolic processes and diseases such as cancer."

Source: Skene D, Raynaud F, Davies S et al. Effect of sleep deprivation on the human metabolome. PNAS. 2014.