Upvote:2
Physical lethargy is not really a problem as long as your mind stays unaffected. But in reality physical lethargy most of the time comes with mental lethargy. There are already lots of answers regarding the 3. hindrance on this SE.
Also see: The Five Mental Hindrances and Their Conquest
As for "bad habits" IMO the best way to deal with them is to be patient. Using willpower and trying to changes things is often a guarantee for more feelings of guilt, failure and suffering. Instead of trying to get rid of you could develop wholesome habits (meditation, sati, virtue of action, speech and thought ...) alongside the bad ones and then step by step replace old habits with new ones.