Any viral illness can cause lethargy,  sleepiness, its can be expected even in post viral state.. Body tolerate a lot of stress during viral illness..  My suggestions is to take Tab Matilda forte 1tab daily for 3months,  plenty of water/liquids and morning walk..  You may feel more sleepy initially after morning walk but it will improve  quality of sleep.. 

This may occur after any illness as it reduces immunity & nutrient stores which need to replenish. We therefore advise rest during viral illness and healthy diet including fruits salads & green vegetables in plenty. This also includes proteins in the form of eggs soyabean germinating gram & moong etc. During illness small amounts are taken frequently due to lack of appetite. Later increase the intake. Start light physical exercise breathing exercise, meditation, stress reduction. You may also take some supplements also like Vitamin C, and Iron if hemoglobin is low which can cause lithargy & sleepiness. You can also personally go to a physician to examine for any other cause & remedy. Viral infections can behave in strange manner in post viral period. But with slow recovery of immunity & effect of virus everything recovers with time. There is no cause for worry. Sleep means your body needs more time & rest to recover from the effects of illness which are difficult to evaluate.