Most adults need approximately 8 hours of sleep in a 24 hours period. When adults get less than 5 hours of sleep within 24 hours peak mental abilities begin to decline.
Sleeping less than 2-3 hours of optimal sleep time (8 hours/24) for several days creates sleep debt.
If sleep debt continues for 5 or more days, people are rarely maximally alert, general performance decreases and particularly cognitive performance become verifiably worse. Most people feel fatigued if experiencing sleep debt.
Not getting enough sleep or sleep of poor quality will create fatigue.