Laziness and sleepiness are not the same thing, you can attribute tiredness to sleepiness but laziness can be somebody that is not tired but don't want to work. It's natural to become tired after a stressful work, at that point the body needs rest and sleep can be a remedy. A lazy person wants to rest and sleep all day when they have things to do and their body can carry them.

If you're talking from a medical point of view I can say that laziness and sleepiness can be because your body is lacking some vital diets. People who don't eat well or their bodies lacks some vital nutrients will not have the energy and willingness to engage in any physical activities. People who are in these conditions can check on Google to know the causes and solutions to their problems or better still see their doctors for prescriptions.