Sleeping pills will knock you out, but overuse can lead to addiction or an inability to sleep without them. Many sleeping medications lead to serious side effects or allergic reactions in some users. Use these only for short-term problems. If you have serious trouble sleeping, visit a doctor and ask about more effective medication.
Melatonin is a safer option, but not as powerful. It's usually most effective at fixing jet lag or another disrupted sleep schedule. It may not be your best bet for a one-time sleep-in. Most over the counter sleeping medications are antihistamines, such as diphenhydramine or doxylamine, sometimes combined with painkillers or alcohol. Side effects such as next-day drowsiness or dizziness are common, and it's unclear whether these are even effective. Prescription-strength drugs are usually benzodiazepines. These can lead to severe addiction and withdrawal symptoms, and will only work while you are taking them. Most other prescription-strength drugs have not been around long enough to gather complete data on effectiveness or side effects. Talk to your doctor about zaleplon (Sonata), zolpidem (Ambien), and eszopiclone (Lunesta) and ask for recent information.