Note: “Sleep late” in the present context refers to people with the evening chronotype who go to bed late AND wake up late compared to the average person (also referred to as OWLS, in the piece below).
Social necessity, or intrinsic drivers such as chronotype, determine when we sleep and when we wake up. People with the Evening Chronotype (Owls) are those who sleep late and wake up late, whereas those with the Morning Chronotype (Larks) are those who sleep early and wake up early. While there are a few who fall into extreme Morning and Evening Chronotype, most of us are somewhere in between (Intermediate Chronotype, Hummingbirds).
The UK Biobank is a large and ongoing study that is intended to tease out the respective contributions of genes and environment (nutrition, lifestyle, exercise, medications etc.) to development of disease. The prospective analysis in this study included 433268 adults aged 38-73 in whom mortality was studied over a period of 6.5 years. Chronotype was determined to fall into 4 groups based on a questionnaire: “Definitely a ‘morning’ type, Rather more a ‘morning’ than an ‘evening’ type, Rather more an ‘evening’ type than a ‘morning’ type, Definitely an ‘evening’ type”. When chronotype was treated as an ordinal variable starting with “Definite Morning Type” and moving to “Definite Evening Type” the study confirmed previous reports showing a significant graded increase in morbidity from psychological disorders, diabetes, neurological disorders, gastrointestinal disorders and respiratory disorders from left (Morning Chronotype) to right (Evening Chronotype). There was a 10% increase in all-cause mortality for Definite Evening Types compared to Definite Morning Types.
Why do people who sleep late suffer this increased morbidity and mortality? The increase in risk has been attributed to these chronotypes consuming less healthy diets with a greater proportion of fat (why they do this is not clearly understood, but it is probably linked to a mismatch between circadian rhythm and food consumption patterns). Previous studies including this one also show an increased risk of diabetes and cardiovascular disease in persons with the Evening Chronotype. Evening Chronotype is also associated with mood and behavioural issues (e.g. a tendency towards substance abuse) that may contribute to increased mortality in this group.
What do Evening-ers do about this? If your chronotype is socially induced (i.e. due work or social engagements that stop you from sleeping early), then it should be possible for you to revert to a hummingbird chronotype simply by adjusting your activities so that you can sleep earlier (exposure to bright light in the morning and taking melatonin before sleep can also help to make this adjustment). If those choices do not work or are unavailable, then the next best thing is to adjust work hours to suit the chronotype.
The bottom line is for Owls to get some early shut-eye and start looking more like the Lark.