Don't have a nightcap before bed tonight. In the short term it seems to work, but if you do it often it can lead to problems.
A nightcap does indeed help some people to fall asleep quickly, the first sleep is deeper and it disrupts your REM sleep. REM sleep is the phase in which you dream and also very important to wake up rested. The latter is therefore the 'problem' of falling asleep with a nightcap.
You may feel fatigued during the day after falling asleep thanks to a nightcap. It can lead to poor concentration and general sleep deprivation. Drinking a nightcap regularly can lead to problems.
It is best not to drink alcohol for two hours before going to sleep. This way you don't become dependent on alcohol to fall asleep.
Do you ever drink a nightcap?