Causes of bodily fatigue, lethargy, and excessive sleepiness are typically attributed to insufficient nighttime sleep, either due to inadequate duration or poor quality.

Adequate nighttime sleep is crucial for the body and brain to rest and recuperate fully.

Without it, individuals may experience daytime sleepiness, fatigue, and overall lethargy.

Another common cause is depression, where individuals often experience profound fatigue, feeling physically and mentally drained, yet may struggle with poor sleep quality or difficulty falling asleep despite feeling tired.

Additionally, systemic chronic illnesses can contribute to bodily fatigue, lethargy, and excessive sleepiness. 

These may include hypothyroidism, sleep apnea syndrome, cerebral ischemia, metabolic encephalopathy, neoplastic tuberculosis, anemia, malnutrition, and heart failure.

Medical evaluation is necessary to identify the specific underlying causes and provide appropriate adjustments and treatments accordingly.