On a visit to TODAY earlier this year, Dr. Carol Ash and Julie Bain, Health Director for Ladies Home Journal, offered tips to help provide answers to that question.
Both Ash and Bain agreed that mental fatigue and stress can be one of the biggest causes of daytime sleepiness.
"Stress is a good thing; it helps us overcome challenges," Ash said. "But sustained stress can lead to anxiety and depression."
These types of stress can keep you awake at night - and exhausted during the day.
Ash advises, when stress and anxiety are keeping you up, write it down, and then confide with someone you trust during the day.
Watch the full TODAY segment below.