How Does Nytol One A Night Work? The active ingredient in Nytol is Diphenhydramine. Diphenhydramine helps to block the normal actions of the chemical histamine. Histamine stimulates the arousal-promoting areas of the brain, Nytol helps to help reduce alertness & help promote drowsiness, leading you into the first stage of sleep which makes it a lot easier to fall into a deep, restful sleep. How To Take Nytol One A Night Tablets Adults; Take one tablet 20 minutes before bedtime. Nytol is not suitable for children under 16 years of age. Do not take this product if you are asthmatic, are pregnant, have a peptic ulcer or have narrow-angle glaucoma. Avoid alcohol. Avoid driving & operating machinery if affected by drowsiness. If this medicine is taken for a prolonged period, you may become tolerant to the effects & it will become less effective at promoting sleep. Consult your doctor if you continue to have trouble sleeping.