A nightcap is a warming alcoholic beverage drunk before going to bed. The nightcap as a drink is an expression that dates back to the 1700s, when folks donned nightcaps as a way of warming themselves and thus promoting better sleep. In a sense an alcoholic nightcap does provide warmth, and also generally makes the drinker a little sleepy. Alcohol at first sedates, and it makes the blood vessels dilate, which moves blood closer to the skin, making the drinker feel warmer.
Another potential origin for the term nightcap is the idea of the “capping” the evening. In earlier times, a person was not considered fully dressed until he or she put on a hat. In other words he or she wasn't finished until capped. Nightcap may thus be used to signify an evening’s finish, and is so used in numerous films where the a young lady invites a gentleman to her apartment or home for a nightcap.
As mentioned, a nightcap can be a drink of any kind, but usually contains alcohol. There are several bartending groups that suggest a perfect recipe for the nightcap, often involving a blend of coffee liqueur or crème de cacao with milk. Some include scotch, whiskey or other hard alcohols. Some contend that drinks like cocoa, chamomile tea, or warmed milk also can be called nightcaps, but tradition dictates an alcoholic beverage, even in fiction. Charles Dickens, in the novel David Copperfield describes Betsy Trotwood taking her nightly drink of wine and water into which she dips strips of toast.
Physicians used to recommend a moderate amount of alcohol before bedtime for those people suffering from difficulty sleeping, but now more is known about the effects of alcohol on the sleep cycle. Though an occasional drink before bed is fine, regularly consuming alcohol at bedtime can lead to sleep difficulties. Alcohol may make people feel drowsy and warm at first, but it also tends to halve the amount of REM sleep they get. It also has a tendency to promote much lighter sleep in the later part of the sleeping time, which means a person who has drunk before bed may wake up more frequently and more quickly move through sleep cycles.