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The reasons for sleepiness could be many. Firstly, could be related to the deficiency in vitamins in body or iron deficiency. This can be easily tested by blood tests. Secondly, hormonal changes can cause sleepiness, like low functioning thyroid gland, which can also be checked on the blood test.
Thirdly, sleep issues like sleep apnea or insomnia can cause daytime sleepiness which will require clinical assessment and sleep study. Finally, low mood or depression commonly causes sleepiness and feeling of exhaustion.