Behavioral and Circadian Rhythms: Best to Be in Synch

Misaligned cycles lead to adverse cardiometabolic consequences in this laboratory study.

We don't feel right when we change our sleep/wake schedule, whether due to shift work, jet lag, daylight savings time, or illness. Humans have daily behavioral cycles (sleep/wake and fasting/eating) and endogenous circadian cycles (e.g., temperature). These researchers examined the metabolic, autonomic, and endocrine effects of disrupting the alignment between these cycles in 10 adults (5 women).

The participants underwent a multiday protocol in an environment free of time cues. Two 24-hour baseline periods with 8-hour sleep opportunities at subjects' regular time were followed by seven 28-hour cycles, with sleep and wake periods scheduled in a 1:2 ratio. Subjects received isocaloric meals every 4 hours in wake periods. Exercise was prohibited.

 In misaligned cycles compared with aligned ones, significant differences included leptin (17% lower), glucose (6% higher), insulin (22% higher), mean arterial blood pressure (3% higher), and sleep efficiency (22% lower). Glucose increases possibly resulted from exaggerated postprandial glucose responses, despite the insulin increases. With maximal misalignment of behavioral and endogenous circadian cycles, leptin suppression was maximal and cortisol levels were inverted (lower at the beginning and higher at the end of a wake episode).

Comment: In a separate essay, Phillips reviews research associating shift work with psychiatric disorders, cancer, glucose intolerance, and hypertension. As the study authors and commentators discuss, the misalignment of behavioral and circadian rhythms might have several potential health consequences. The decreases in leptin (which stimulates appetite and signals energy deficit) and increases in insulin were insufficient to control glucose levels, simulating the insulin-resistant phenotype. Increases in blood pressure could predispose to hypertension and cardiovascular disease. These changes would be transitory in jet-lagged travelers, longer-lasting for night-shift workers, and potentially persistent for people with unusual biological clocks. Phillips suggests that each individual has an optimal schedule. Unfortunately, reality can get in the way of our lifestyle being in synchrony with our biorhythm.