There are several possible reasons why a person may be tired. First and foremost, a lack of sleep can be the most obvious. However, not everyone realizes that they are lacking in sleep as they do go to bed at a reasonable hour, rise at a reasonable hour and so on. It could be that the quality of sleep is poor.
Sleeping with lights on prevents a person from getting sufficient “deep sleep” when there should be non- REM (Rapid Eye Movement). Deep sleep is essential for the brain to recover from daily activities and to prepare for the new day. Inadequate deep sleep can make a person feel tired on waking. When awakened from a deep sleep (rather than other periods of sleep), a person is likely to be disoriented and sluggish. Stress affects sleeping patterns and may prevent a person from obtaining sufficient deep sleep.
Poor eating habits and spikes in blood-sugar cause tiredness. Eating lots of sugary snacks has a detrimental effect on the system and although initially energy levels may increase, they are short-lived and an overwhelming tiredness can result. Unfortunately, a cycle begins as the sufferer tries to remedy this tiredness through caffeine or more sugary snacks and the sugar "highs" and "lows" become characteristic.
Being unfit can make a person feel tired as can being overweight which will also contribute to feelings of tiredness as a person exercises less and less and a lack of motivation is mistaken for "real" fatigue. Ironically, the less a person exercises, the more tired he or she will feel! Even being dehydrated causes tiredness and people are often not aware that they are dehydrated so do not treat the symptom of tiredness effectively.
There are some medical issues that could be the cause of tiredness such as Type 2 Diabetes or an underactive Thyroid. Tiredness is a symptom of these. Sleep apnoea, which blocks the airways, thereby interrupting sleep as the person wakes briefly to take a breath, is also a cause of tiredness.
However, it is the mostly easily remedied reasons that cause tiredness.