Common hangover symptoms include fatigue and weakness, thirst, head and muscle aches, stomach pain and poor sleep. Increased sensitivity to light and sound, shakiness, decreased focus, and mood issues such as anxiety, irritability, and depression are also symptoms of a hangover, according to Mayo Clinic.
Hangover symptoms usually begin within several hours after consuming alcohol when the blood alcohol concentration levels begin to fail, peaking when the BAC falls to zero, claims to About.com. Hangover symptoms typically last up to 24 hours.
As of 2014, researchers do not fully understand what causes hangovers, although some believe that it is due to alcohol interfering with the body's natural chemical balance, according to the Smithsonian. Another theory is that the by-product of alcohol, acetaldehyde, is more toxic than alcohol itself, causing hangover symptoms.