According to the "Merck Manual Home Health Handbook," sleep can be disturbed by many things such as stress or diet. Lack of sleep makes people tired throughout the day and unable to perform optimally. Over-the-counter sleep aids help when sleep does not come easily and can be used for up to two weeks to help get the recommended amount of sleep, which is 6 to 10 hours each night. Unisom makes sleep aids for adults, and has three types, each with one active ingredient.

According to Unisom, SleepTabs help people fall asleep an average of 23 minutes faster than without Unisom. The active ingredient in SleepTabs is 25 mg of doxylamine succinate. Drugs.com lists doxylamine succinate as an antihistamine and sedative-hypnotic. A sedative-hypnotic is a chemical used to induce sleep by relaxing the mind and body.

Doxylamine used as an antihistamine comes in dosages of 4mg for an adult, with dosages as low as 6mg able to produce a sedative effect. Its use as a sleep aid is due to the amount used in each pill, 25mg, which increases the sedative-hypnotic properties.

Since SleepTabs are listed as a sedative-hypnotic, they are recommended for adult use only, with one pill taken 30 minutes prior to bedtime.

Diphenhydramine HCl is the main active ingredient in Unisom's SleepGels. Diphenhydramine HCl is an antihistamine with sedative effects and is used for many things other than for allergies. Diphenhydramine, depending on the dosage and combination with other drugs, is used for motion sickness and as a sleep aid. Diphenhydramine is an anticholinergic, which means it helps reduce the excitability and muscle responses of the central nervous system that can prohibit relaxation and a good night’s sleep.

Drugs.com says diphenhydramine HCl begins working in about an hour and remains in the bloodstream for up to six hours and should be taken with water, when in pill form.

One SleepGels caplet is the recommended dosage, as it contains 50 mg of diphenhydramine HCI, and must be taken with water. Drugs.com advises diphenhydramine should be taken by adults, and to avoid other sedatives like alcohol when taking a sleep aid.

SleepMelts contain 25 mg of diphenhydramine HCl in each tablet, with a recommended dosage of two tablets for an adult. Diphenhydramine is an antihistamine used as a sleep aid.

SleepMelts are tablets that dissolve in the mouth. With a cherry flavor, the tablets dissolve on the tongue, or under the tongue, allowing the diphenhydramine HCl to enter the bloodstream sublingually. Since the sublingual gland, located under the tongue, is a salivary gland, it is a more direct route to the bloodstream.

Medications given by sublingual administration have advantages over oral administration. They do not require water and get the medication into the body more quickly, allowing the sleep aid to begin working faster.