Exhaustion Due to Heat This is mostly due to the fact that taller (and heavier) persons create more body heat. If they produce it quicker than they can eliminate it, like during hard activity, this might result in heat stroke or heat exhaustion. These conditions are usually recognized by symptoms such as headache, fatigue, hot flashes, and dizziness. Tiredness From Work This is probably the most common cause of daytime fatigue for people who do not suffer from sleep disorders. For example, someone who works on a construction site all day long will be very tired after work because they don't get enough rest. People who work in jobs that require them to move around a lot also experience daytime fatigue because they aren't getting adequate recovery time between tasks. Nighttime Fatigue This type of fatigue occurs when you wake up feeling exhausted even though it was supposed to be your day off. Most likely, you have become used to sleeping eight hours per night but since your body is still processing everything that happened during the day, you need more sleep. Some people who suffer from insomnia may find relief in a caffeine-free diet drink or energy bar before bed. Drowsiness While Sitting For many people, sitting for too long can lead to feelings of exhaustion. This is especially true if you are used to standing or walking much of the day.