Does a late bedtime make you fatter?

On 30th June 2021, a new study was published in the JAMA journal for public health, looking at the association between sleep, bedtime and obesity. The study delivered some encouraging results for those of us who like our sleep and go to bed before midnight.

The study:

The study included adults between the age of 35 and 67 and analysed their length of sleep, time of waking and bedtime, and daytime napping. This was then investigated alongside the prevalence of obesity (defined as a BMI over 30) and abdominal obesity (a waist circumference over 102 cm for men and 88 cm for women).

In total the study looked at 136,652 adults with an average age of 51. 19.9% of the participants were classified as obese with 27.1% having abdominal obesity. The average sleep time was 7.8 hours per night, with 39.7% of the participants reporting they frequently had a daytime nap. The mean bedtime was 10:54 pm in high income countries, 10:21 pm in middle income countries and 10:09 pm in low income countries.

The results are compelling:

Where the study gets interesting is the association between obesity, length of sleep and bedtime. The results showed that individuals going to bed between 8 and 10pm showed the lowest prevalence of obesity, obesity was 9% more common for a 10pm to 12am bedtime, 20% more common for a 12am to 2am bedtime and 53% higher for 2am to 6am. This is especially significant for shift workers whose jobs, by their very nature, often require late bedtimes or reverse sleeping patterns, and previous evidence has already shown significant health risks associated with night shift working.

When looking specifically at length of sleep, those individuals who consistently had 5 hours or less sleep per night were 27% more likely to be obese, whereas there appeared to be no link with obesity in those sleeping for over 6 hours per night.

Interestingly, daytime napping seemed to have no bearing on obesity rates.

Breaking bad habits:

So, in summary, it appears that there is a significant correlation between length of sleep, bed time and obesity. The current guidelines recommend at least 7 hours sleep per night, but do not specify a bedtime. This may need to be reconsidered in the light of the study’s finding. The study noted that people with late bedtimes were more likely to be urbanites, smokers and drinkers who consumed more calories but exercised less.  Perhaps an earlier bedtime would help break some of these bad habits, including less time for bedtime snacking.

Words of wisdom:

“Sleep is that golden chain that ties health and our bodies together ” – Thomas Dekker