morning after, the - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The unpleasant results of an earlier activity, especially overindulgence in alcohol. For example, A headache is just one of the symptoms of the morning after. This expression originated in the late 1800s as a synonym for a hangover (and was often put as the morning after the night before). By the mid-1900s, however, it was also being used more loosely for the aftereffects of staying up late.