Night owls who definitely prefer evenings over mornings have a 10% higher risk of all-cause mortality compared with those who definitely prefer mornings, according to a large prospective cohort study from the UK conducted over 6.5 years.
Definite evening chronotypes are also at greater risk for a wide range of diseases and disorders than definite morning chronotypes, the same analysis indicates.
“Increased eveningness, particularly definite evening type, was associated with increased prevalence of a wide variety of diseases or disorders, including diabetes, psychological, neurological, respiratory, and gastrointestinal/abdominal disorders,” write Kristen Knutson, PhD, Northwestern University, Chicago, Illinois, and Malcolm von Schantz, PhD, University of Surrey, Guildford, UK.
The study was published online April 11 in Chronobiology International.
“Further, increased eveningness was significantly associated with increased risk of all-cause mortality over 6.5 years,” they add, although this was only significantly higher in the age group of 63 to 73 years.
“Identifying novel, potentially modifiable lifestyle factors associated with increased morbidity and mortality can lead to innovative strategies for improving health,” they suggest.
Misalignment Between Endogenous Body Clocks and Social Activities
The researchers say that previous studies in this field have focused on the higher rates of metabolic dysfunction and cardiovascular disease, but this is the first to look at mortality risk.
The analysis was based on 433,268 participants from the UK Biobank, a large, prospective, population-based study. Participants ranged in age from 38 to 73 years, with a mean age of 56.5 years.
“Approximately 27% identified as definite morning types, 35% as moderate morning types, 28% as moderate evening types, and 9% as definite evening types,” the researchers observe. Each “incremental increase in eveningness from definite morning to definite evening type was associated with increased odds of having each comorbidity,” they note.
Association Between Comorbidities and Chronotype
| Comorbidity | Definite morning | Definite evening | P |
|---|---|---|---|
| Psychological disorders | Reference | 1.94 | < .001 |
| Diabetes | Reference | 1.30 | < .001 |
| Neurological disorders | Reference | 1.25 | < .001 |
| Gastrointestinal/abdominal disorders | Reference | 1.23 | < .001 |
| Respiratory disorders | Reference | 1.22 | < .001 |
Researchers also looked at the risk for all-cause mortality as well as death from cardiovascular disease (CVD) in association with chronotype, and adjusted for potential confounders.
“Chronotype as an ordinal variable was associated with all-cause mortality (1.02 per level, P = .017) and CVD mortality (1.04 per level, P = .06), such that increasing eveningness was associated with greater mortality,” Knutson and von Schantz report.
The association between all-cause and CVD death was also observed in both genders.
However, the association between chronotype and all-cause mortality was significant only for participants between 63 and 73 years of age (P = .006) and not among the younger age groups.
In contrast, neither moderate morning nor moderate evening chronotypes were at increased risk of all-cause mortality.
“Mortality is a significant clinical outcome and any increase in age-adjusted risk of death warrants attention,” the researchers stress.
“The health of evening types could be compromised by misalignment between their endogenous biological clocks and the timing of social activities (eg, work or meals), termed circadian misalignment,” Knutson and von Schantz suggest.
Adjusting Schedules, Where Possible, Might Help: Can Owls Become Larks?
The authors have previously reported that genetics and environment play approximately equal roles in whether people are morning or night types, or somewhere in between.
“You’re not doomed,” Knutson said in a press release by Northwestern University. “Part of it you don’t have any control over and part of it you might.”
“It could be that people who are up late have an internal biological clock that doesn’t match their external environment. It could be psychological stress, eating at the wrong time for their body, not exercising enough, not sleeping enough, being awake at night by yourself, maybe drug or alcohol use. There are a whole variety of unhealthy behaviors related to being up late in the dark by yourself,” she says.
The researchers propose that at least some of the environmental determinates of eveningness could be modifiable, for example by use of light in the morning and melatonin in the evening.
Adjusting work schedules to better fit with individual chronotypes might also help improve the health of evening types.
“If we can recognize these chronotypes are, in part, genetically determined and not just a character flaw, jobs and work hours could have more flexibility for owls,” Knutson said. “They shouldn’t be forced to get up for an 8 am shift. Make work shifts match peoples’ chronotypes. Some people may be better suited to night shifts.”
The researchers also point out that daylight savings time is especially difficult for night owls who are already struggling with the impositions of a society that dictates when people must start work each day.
“There are already reports of a higher incidence of heart attacks following the switch to summer time,” says von Schantz in the press release.
“This is a public health issue that can no longer be ignored. We should discuss allowing evening types to start and finish work later, where practical. And we need more research about how we can help evening types cope with the higher effort of keeping their body clock in synchrony with sun time.”
In future research, Knutson and colleagues want to test an intervention with night owls to get them to shift their body clocks to adapt to an earlier schedule. “Then we’ll see if we get improvements in blood pressure and overall health,” Knutson observed.
The authors have reported no relevant financial relationships.
Chronobiol Int. Published online April 11, 2018. Abstract
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