Why you should go to sleep at the same time all week

Small differences in sleeping habits between work and rest days could lead to unhealthy changes to the bacteria in our guts, a study suggests.

This may be partly a result of people with "social jetlag" having slightly poorer diets, the UK researchers found.

Heavily-disrupted sleep, particularly shift work, is known to have a negative impact on health.

Keeping bed times and wake times consistent and eating a balanced diet may help reduce our risk of disease.

The study of nearly 1,000 adults by Kings College London scientists found that even a 90-minute difference in the midpoint of your night's sleep over the course of a normal week could influence the types of bacteria found in the human gut.

Having a wide range of different species of bacteria in your digestive system is really important. Some are better than others, but getting the right mix is key to preventing a number of diseases.

"[Social jetlag] can encourage microbiota species which have unfavourable associations with your health," said Kate Bermingham, study author and senior nutrition scientist at health science company Zoe.

Going to sleep and waking up at very different times during the week, compared to the weekend, is known as having social jetlag.

It is thought to affect more than 40% of the UK population, the study says, and is most common in teenagers and young adults, then tapers off as we age.

Participants in this study, in the European Journal of Nutrition, had their sleep and blood analysed, stool samples collected and recorded everything they ate in a food questionnaire.

Those who had social jetlag (16%) were more likely to eat a diet laden with potatoes, including crisps and chips, plus sugary drinks, and less fruit and nuts.

Previous research showed people with social jetlag ate less fibre than those with more consistent sleeping times. Other studies found social jetlag was linked to weight gain, illness and mental fatigue.

"Poor quality sleep impacts choices - and people crave higher carb or sugary foods," says Dr Bermingham.

An unhealthy diet can then affect levels of specific bacteria in the gut.

The researchers found that three out of the six microbiota species which were more plentiful in the guts of the social jetlag group are linked to poor diet quality, obesity and higher levels of inflammation and stroke risk.

The relationship between sleep, diet and gut bacteria is complicated and there is still a lot more to find out, the research team says.

In the meantime, their advice to keep things consistent, if you can, over the course of a week.

"Maintaining regular sleep patterns, so when we go to bed and when we wake each day, is an easily adjustable lifestyle behaviour we can all do, that may impact your health via your gut microbiome for the better," says Dr Sarah Berry, from King's College London.

The NHS website recommends you try to:

Gaining weight? You may need more sleep

A daytime nap is good for the brain

More than half your body is not human

Gut instinct: Why I put my poo in the post

How bacteria are changing your mood

How quickly can you improve your gut bacteria? BBC Food

Why a workout is good for your gut bacteria - BBC Future