Monday November 18 2013
Some shop-bought coffees contain up to 500mg of caffeine
"Drinking even one strong coffee in the afternoon can knock an hour off your sleep," Mail Online reports. The headline is based on a small study that tested the effects of a 400mg caffeine pill taken either at bedtime, or three or six hours before.
Researchers found that the caffeine dose (similar to that of a large shop-bought coffee) appeared to disrupt sleep even when taken six hours before bedtime. The caffeine reduced the total amount of time volunteers slept by about an hour. This effect was seen regardless of when the caffeine pill was taken.
It is important to bear in mind that the study only included a very small number of people, all of whom were generally healthy and did not have problems with sleep.
The findings need to be confirmed in larger studies with more mixed groups of people of different ages to be sure of exactly how long the effects of caffeine last.
Overall, the study suggests that caffeine may have an effect up to six hours before going to bed. For people concerned about having a good night's sleep, it is probably best to avoid caffeine close to when you go to bed.
Where did the story come from?
The study was carried out by researchers from the Henry Ford Hospital and Wayne State College of Medicine in Detroit, and Zeo Inc, a company that works in the area of sleep and produces sleep monitoring devices.
It was funded by Zeo Inc. The study does not make any recommendations about the use of sleep monitoring devices, so it appears there is no direct conflict of interest from this perspective.
The study was published in the peer-reviewed Journal of Clinical Sleep Medicine.
Mail Online's reporting of the study is accurate.
What kind of research was this?
This was a randomised controlled trial assessing the effect of caffeine on sleep quality when taken at different times before sleep. The effect of caffeine consumption before bedtime on sleep is well known. As a result it is recommended that people who want a decent night's sleep should avoid caffeine close to bedtime.
However, the researchers reported that no studies have looked at the effects of a set dose of caffeine at different times before trying to go to sleep.
What did the research involve?
Participants in the study were given caffeine or placebo pills either just before they would usually go to bed, or three or six hours before. The researchers assessed how these treatments affected the participants' sleep.
The participants were 16 healthy adults who did not have problems with sleep and did not work night shifts. They were recruited by a local advert and compensated for their participation.
Potential participants filled in sleep diaries the week before the study and only people who slept between six-and-a-half and nine hours who fell asleep within half an hour of going to bed were recruited.
People with a history of past psychiatric illness, with any current medical illness or who were taking certain types of medications were also excluded.
The participants were also asked about their usual daily and weekly caffeine consumption in the form of tea, coffee, soft drinks, energy drinks or chocolate. People who drank more than five caffeinated beverages a day were excluded.
The caffeine pills contained 400mg of caffeine. The researchers reported that an 8 fluid ounce (fl oz) cup of home-brewed coffee was estimated to contain 100mg of caffeine, while a 16 fl oz (0.8 of a pint) cup of commercially prepared brewed coffee can contain up to 500mg of caffeine.
Participants were asked not to drink caffeinated beverages or alcohol after 4pm during the study. They were also asked to maintain a fixed bedtime and waking time.
All participants took the study pills every other night for four nights. They took three pills on these nights at six hours before their usual bedtime, three hours before bedtime, and at bedtime. On one night all three pills were placebo, while on the other nights one of the pills contained caffeine. The time at which the caffeine pill was taken was determined at random, and each person took a caffeine pill at each of the three time slots on one night during the study.
Participants wore a sleep monitoring headband at night, which measured:
- total sleep time
- the time it took for the person to enter persistent sleep
- wake time during the night
- sleep efficiency (the number of minutes of sleep divided by the number of minutes in bed)
On each morning during the study, the participants completed a sleep diary to record, for example, if they had any difficulty going to sleep and how they slept.
The researchers then looked at how the timing of caffeine intake affected self-reported and objectively measured (headband recorder) sleep.
What were the basic results?
Of the 16 participants, six men and six women (average age 29.3 years) completed the study correctly and their data was analysed. On average, these participants usually consumed 115mg caffeine a day. The amount of caffeine they consumed did not vary significantly on the different study days.
Based on the objective (headband) measurements, caffeine taken at bedtime, or three or six hours before bedtime significantly reduced total sleep time compared with placebo. The reduction in sleep time with caffeine was about an hour. Caffeine did affect some of the other objective sleep measures, but these differences were not always statistically significant.
Compared with placebo, caffeine had the greatest effect on self-reported sleep if taken at bedtime or three hours before bedtime. Taken at these time points, caffeine:
- significantly reduced total sleep time
- significantly increased the time taken to fall asleep
When people took caffeine six hours before bedtime, they reported that:
- they slept on average 41 minutes less than with placebo
- they took about twice as long to fall asleep compared with placebo
However, these differences were not as large as those seen when caffeine was taken closer to bedtime, and were not large enough to be statistically significant.
Caffeine did not affect self-reported waking time during the night, sleep quality or sleep efficiency.
How did the researchers interpret the results?
The researchers concluded that even if taken six hours before bedtime, a moderate dose of caffeine has important disruptive effects on sleep. They say that this supports recommendations to avoid substantial caffeine consumption for a minimum of six hours before going to bed.
This study suggests that caffeine intake may affect sleep even if taken six hours before bedtime. The main strengths of the study were the use of a randomised and blinded design, and the use of both self-reported and objective measures of sleep.
However, there are also limitations to the study:
- The study was very small and included a very select group of participants. It analysed data from only 12 healthy young to middle-aged adults who took each of the timed test doses of caffeine on one night only. Larger studies in more mixed populations would be needed to confirm the findings and see if they apply to other groups.
- Not all of the self-reported and objective measurements of sleep completely agreed. For example, caffeine taken six hours before sleep only had a statistically significant effect on the objective measure of total sleep time, but not self-reported sleep time. The researchers suggest that this difference may be as a result of people having broken sleep, which they notice less than if they take longer to fall asleep, for example. Larger studies where people undergo more extensive measurements in a sleep lab may help confirm the effects.
- The average caffeine consumption of participants was about 100mg per day – about one home-brewed 8 fl oz cup of coffee. Other studies would be needed to see if the effect of the caffeine dose used in the study (400mg) differed in people who were used to consuming more or less caffeine.
Despite these limitations, if you are having problems sleeping, it makes sense to try limiting your consumption of stimulants, such as foods and drinks that contain caffeine, especially in the evening, to see if this helps.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.