Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle

Abstract

Summary The electric light is one of the most important human inventions. Sleep and other daily rhythms in physiology and behavior however, evolved in the natural light-dark cycle[1] and electrical lighting is thought to have disrupted these rhythms. Yet how much the age of electrical lighting has altered the human circadian clock is unknown. Here we show that electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day, increased light exposure after sunset, and a delayed timing of the circadian clock as compared to a summer natural 14h40min:9h20min light-dark cycle camping. Furthermore, we find that after exposure to only natural light, the internal circadian clock synchronizes to solar time such that the beginning of the internal biological night occurs at sunset and the end of the internal biological night occurs before wake time just after sunrise. In addition, we find that later chronotypes show larger circadian advances when exposed to only natural light, making the timing of their internal clocks in relation to the light-dark cycle more similar to earlier chronotypes. These findings have important implications for understanding how modern light exposure patterns contribute to late sleep schedules and may disrupt sleep and circadian clocks.

Chronotype and environmental light exposure in a student population

“…Significantly, we did not find that light intensity was associated with chronotype. Initially, these findings appear to contradict earlier studies where increased light exposure is associated with an earlier chronotype in the general population (Roenneberg and Merrow 2007; Wright et al 2013; Roenneberg et al 2015; Stothard et al 2017). However, our findings are derived exclusively from a university student population aged between 17 and 26 years.…”

“…A recent study compared the same individuals living under their normal urban routines (including artificial light at night) with a period under natural light exposure (camping without artificial light). The findings demonstrated that increased exposure to natural light advanced the circadian phase of all individuals (Wright et al 2013; Stothard et al 2017). Increasing photic zeitgeber strength by spending more time outside has also been correlated with self-reported chronotype: the more time spent outside, the earlier the chronotype (Roenneberg and Merrow 2007; Roenneberg et al 2015).…”

Impact of circadian misalignment on energy metabolism during simulated nightshift work

“…The circadian time-keeping system in humans modulates energy metabolism so that wakefulness, activity, and food intake are promoted during the solar day and sleep, inactivity, and fasting occur during the solar night (2). With the widespread use of electrical lighting, however, work and social activities are capable of being extended further into the night (6,7). Being awake during the biological night leads to disturbed physiology and behavior, because it creates a state of desynchrony between the circadian clock and wakefulness-sleep cycle known as circadian misalignment.…”

A matter of timing

“…Although the people were only retiring to their sleeping bags an hour or so earlier than usual, their melatonin levels started to rise much earlier, around sunset. And rather than lingering in the morning, melatonin levels fell before people woke up (8). "Humans are like other species," says Wright.…”