Dr. Andrew Lim, University of Toronto – Night Owls and Morning People

Feb 6, 2013

In today’s Academic Minute, Dr. Andrew Lim of the University of Toronto explains the genetics behind the human sleep cycle.

Andrew Lim is an assistant professor in the Department of Medicine at the University of Toronto and a neurologist at Sunnybrook Health Sciences Centre, Toronto, Canada. His research is focused on using statistical, genetic, neuropathological, and epidemiological approaches to understand the genes and neural circuits that regulate biological clocks and sleep. He holds an M.D. from the University of Toronto.

About Dr. Lim

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Dr. Andrew Lim – Night Owls and Morning People

An internal biological clock regulates the timing of 24-hour rhythms in a variety of human biological functions and attributes including sleep, appetite, cognition, and susceptibility to diseases like heart attacks.  The timing of these circadian rhythms can vary dramatically between individuals, with some people being “morning larks” while others are “night owls”.  Moreover these patterns run in families, suggesting a genetic influence.  

We sought to identify gene variants influencing human circadian rhythms.  We measured 1 week of sleep and activity rhythms in 537 older individuals using wristwatch-like devices, obtained DNA, and examined genomic sites that differ between individuals.  We found a variant near the Period 1 gene, a gene similar to one regulating circadian rhythms in fruit flies, that was associated with individuals’ sleep and activity rhythms.  Each individual has one of 3 possible versions of this variant.  36% have the AA or early version, 48% the AG or intermediate version, and 16% the GG or late version.  Individuals with the late version had sleep and activity rhythms on average 1 hour later than those with the early version.  Among participants who subsequently died, those with the late version were most likely to die at or around 5:50 in the evening, while those with the early or intermediate version were most likely to die at or around 10:50 in the morning.

This work reinforces the idea that circadian rhythms are not merely a matter of choice or environment, but are partly genetically determined.  It also emphasizes that these rhythms have an important influence on medical diseases leading to death.  It opens the door to optimizing human performance by individualizing work, school, travel, and other schedules based on one’s genes.  It also raises the possibility of individualizing the timing of medical interventions to protect patients during the time of day they are most likely to die.