A nightcap is a drink taken shortly before bedtime. For example, a small alcoholic beverage or glass of warm milk can supposedly promote a good night's sleep.[1][2]
^Stone, Barbara M. (June 1980). "Sleep and low doses of alcohol". Electroencephalography and Clinical Neurophysiology. 48 (6): 706–709. doi:10.1016/0013-4694(80)90427-7. PMID6155259.