A team at the In­sti­tute of Can­cer Re­search in Lon­don found women had larg­er waist­lines if their bed­room was "light enough to see across" at night.

How­ev­er, they cau­tion there is not enough ev­i­dence to ad­vise peo­ple to buy thick­er cur­tains or turn off lights.

The study of 113,000 women was pub­lished in the Amer­i­can Jour­nal of Epi­demi­ol­o­gy.

The women were asked to rate the amount of light in their bed­rooms at night as:

�2 Light enough to read

�2 Light enough to see across the room, but not read

�2 Light enough to see your hand in front of you, but not across the room

�2 Too dark to see your hand or you wear a mask

Their an­swers were com­pared to sev­er­al mea­sures of obe­si­ty. Body Mass In­dex, waist-to-hip ra­tio and waist cir­cum­fer­ence were all high­er in women with lighter rooms.

Prof An­tho­ny Swerd­low, from the In­sti­tute of Can­cer Re­search, told the BBC:

"In this very large group of peo­ple there is an as­so­ci­a­tion be­tween re­port­ed light ex­po­sure at night and over­weight and obe­si­ty.

"But there is not suf­fi­cient ev­i­dence to know if mak­ing your room dark­er would make any dif­fer­ence to your weight.

"There might be oth­er ex­pla­na­tions for the as­so­ci­a­tion, but the find­ings are in­trigu­ing enough to war­rant fur­ther sci­en­tif­ic in­ves­ti­ga­tion."

One pos­si­ble ex­pla­na­tion is that the light is dis­rupt­ing the body clock, which stems from our evo­lu­tion­ary past when we were ac­tive when it was light in the day and rest­ing when it was dark at night.

Light al­ters mood, phys­i­cal strength and even the way we process food in a 24-hour cy­cle.

Ar­ti­fi­cial light is known to dis­rupt the body clock by de­lay­ing the pro­duc­tion of the sleep hor­mone mela­tonin.

Prof Derk-Jan Dijk, from the Sur­rey Sleep Cen­tre, said there would be no harm in try­ing to make bed­rooms dark­er.

He told the BBC: "Peo­ple in gen­er­al are not aware of the light present in their bed­room, I think peo­ple should as­sess their bed­room and see how easy it would be to make it dark­er."

Street lights, some alarm clocks and stand­by lights on elec­tri­cal equip­ment such as tele­vi­sions could light a room, he said.

"Over­all this study points to the im­por­tance of dark­ness," he con­clud­ed.

The study was fund­ed by Break­through Breast Can­cer and the find­ings emerged from a long-term study to un­der­stand the risk fac­tors for breast can­cer.

Obe­si­ty is known to in­crease the odds of the dis­ease.

Dr Matthew Lam, from the char­i­ty, com­ment­ed: "It's too ear­ly to sug­gest that sleep­ing in the dark will help pre­vent obe­si­ty, a known risk fac­tor for breast can­cer, but the as­so­ci­a­tion is cer­tain­ly in­ter­est­ing.

"Whilst we are learn­ing more and more each day about the en­vi­ron­men­tal, ge­net­ic and lifestyle fac­tors that af­fect breast can­cer risk, it is not yet pos­si­ble to pre­dict who will get breast can­cer, and for women who have been di­ag­nosed with the dis­ease, we can't yet say what caused it." (BBC)

What is obe­si­ty?

A per­son is con­sid­ered obese if they are very over­weight with a high de­gree of body fat.

Some ex­perts be­lieve obe­si­ty is re­spon­si­ble for more ill health than smok­ing. Be­ing sig­nif­i­cant­ly over­weight is linked to a wide range of health prob­lems, in­clud­ing:

Di­a­betes

Heart dis­ease

High blood pres­sure

Arthri­tis

In­di­ges­tion

Gall­stones

Some can­cers (eg, breast and prostate can­cers)

Snor­ing and sleep ap­noea

Stress, anx­i­ety, and de­pres­sion

In­fer­til­i­ty

The most com­mon way to as­sess if a per­son is obese is to check their body mass in­dex. BMI is cal­cu­lat­ed by di­vid­ing your weight in kilo­grams by your height in me­tres squared.

If your BMI is above 25 you are over­weight. A BMI of 30-40 is con­sid­ered obese, while above 40 is very obese. A BMI of less than 18.5 is un­der­weight.

An­oth­er use­ful method is to take a waist mea­sure­ment be­cause fat in the cen­tre of the body (ap­ple-shaped obe­si­ty) is much more strong­ly linked to health risks than fat more wide­ly dis­trib­uted on the arms and legs. Women with a waist of 80cm or greater and men with a waist of 94cm or greater are more like­ly to de­vel­op obe­si­ty-re­lat­ed health prob­lems.

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