The word nightcap originally comes from the cloth cap which was put on before bed to keep a person warm on cold nights. In this case, though, we're talking about a drink taken shortly before bedtime which is supposed to help you sleep.
In a new study, a group of US researchers have looked at the effects of consecutive nights of presleep alcohol on our sleep. Body clock expert Prof Annie Curtis from the RCSI joined the Drivetime show on RTÉ Radio 1 to discuss the report. (This piece includes excerpts from the conversation which have been edited for length and clarity).
So are nightcaps good or bad for you? "Alcohol before you go to bed allows you to go to sleep quicker, and that's one of the reasons why many people do it", points out Curtis. "The only problem is it's been shown that nightcaps are associated with fatigue, decreased working memory, cognitive performance the next day. And if people are feeling like that the next day, they might think 'oh, do you know what? I need to make sure I have a good night's sleep. I'll have another nightcap.'
"Data from the US shows 20% of adults use a nightcap as a sleep aid, 30% if you have insomnia. What this study did was really try to understand what a nightcap is doing to people's sleep. There's been lots of studies done on this. In previous studies, they were just looking at the effect of, say, one drink and one night's sleep. In this study, they looked at the effect of consecutive nights of nightcapping, let's call it that."
Having an alcohol drink before turning in for the night affects both our non-REM and REM sleep. "If you think of your brain as a big warehouse, non-REM sleep is a foreman putting all the memories of the day into the filing cabinets", explains Curtis. "We tend to have more non-REM sleep at the start of our sleep cycle, so say you go to bed at 10 or 11, you tend to have more cycles of non-REM until about maybe say two or three in the morning.
"REM sleep is a different thing and that's rapid eye movement. If non-REM is about storing memories, REM is about somebody coming into that warehouse, taking out all those files, looking at all those memories and making sense of them and making insights and connections between all of them. REM sleep is really the creative part of our sleep."
What the researchers found is that nightcapping increased our non-REM sleep but decreased the REM, which is problematic, says Curtis. "Not only is it all about creativity and all of that, but REM is really important for your memory and emotional regulation. If you don't have enough REM sleep, you act like a toddler. You're emotionally not very attuned to what's going on. You can be a little bit dysfunctional, you can lash out at people, that sort of thing, so that's why REM is really important. So what they showed was a decrease in the REM if you had continual nightcaps."
Does it matter what time you have your nightcap - or is it any alcohol consumption going to cause this problem? "The REM stage happens in that second half of your sleep, and the fact that you're reducing the amount of REM, you are more likely to wake up and stay up. The effect on sleep is very pronounced when you have those nightcaps just before you go to sleep because you haven't metabolized the alcohol, so one way to get around it is maybe have a lunch cap instead of a nightcap."
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The views expressed here are those of the author and do not represent or reflect the views of RTÉ
