To answer your questions specifically:
“I mean, would sleeping be a sin?”
If you have, for example, the opportunity to go to Mass, but instead you sleep for 10 hours and don’t go to Mass, then, yes that is a venial sin. If nothing else, it is a sin of laziness (10 hours of sleep). I confess if I don’t get to Mass in the morning due to not waking up when my alarm goes off. That is definitely simply laziness. If you sleep through your alarm by accident, then it would not necessarily be a sin. As far as sleeping, I don’t know if you can really have a “set amount” of hours which makes it sinful, but if you are not sick, I would think that 12 hours would certainly be a sin, regardless of the circumstances (except, of course, if you were up all night or some similar occurance the night before, because then sleeping 12 hrs in one night and none the night before adds up to 6 hrs each night). As a previous person said, you must use prudence. The more involved you become, the more you desire to live only for God and not for yourself, and the more holy you become, the more things you will accuse yourself of. The holiest Saints confessed the most often. For example, the holier you are, the more likely it is you will commit a sin of omission, for if you are slipping backwards out of your daily habits (or simply maintaining lukewarm ones), you sin venially at least. Also, a cleric is held to a higher level than a secular. What may be merely an imperfection or nothing at all wrong for a secular could be a mortal sin of omission for a cleric, especially for the Pope, the Vicar of Christ, who, by the very dignity of his office, is required to have the purifity of the angels.
"Sitting and watching TV?"
Just about everything on TV nowadays is sinful to watch. I suppose not all, but the very vast majority. If you decide: I like such and such a program (that is, considering, of course, that it is not a sinful program in the first place), and then you decide: I will watch this 30 minute show at 8 o’clock, then that is not a sin. It is not a sin to have recreation. But, if you sit on the couch and just flip around or stare at the screen simply because you have nothing else to do, not with any prupose of watching a certain show, just flipping through channels until you “find something”, it would most likely be a sin, since it inevitably comes from a laziness of sorts. Moreover, watching TV more than one prays, attends Mass, etc. (although many will say to the contrary), is a sin. That is putting something about God and His Holy Church.
Basically, instead of watching TV or doing little things that you think might be sinful (wasting time may be one of the greatest sins and occasions of sin), instead get a good Catholic book–be it for holy reading (lives of the Saints, Imitation of Christ, etc.) or a prayer book or a history of the Church–and read it instead of doing whatever else you might be doing.
If you want to study doctrine and know what the Church teaches in a very simple and clear way, I would suggest the Baltimore Catechism. There are very many degrees of specificity and difficulty. I would suggest if you know very little to start on Baltimore Catechism 2, then work through 3, then Baltimore Catechism 4 (also “Explanation of the Baltimore Catechism”, which is for teachers of catechism). The Baltimore Catechism is not a proclaimed infallible dogma of the Church (no catechism is, unless all it quotes is infallible statements), so there is one problem in it. The part where it talks about there being 3 Baptisms is clearly wrong, in the words of St. Paul in his eistle to the Galations: ONE Lord, ONE Faith, ONE BAPTISM. We know this from the Council of Trent, Canons on Baptism: “If anyone saith that true and natural water is not necessary for Baptism and thus WRESTS into some metaphor the words of our Lord Jesus Christ: ‘unles a man be born again of water and the Holy Ghost’ [Jn. 3.5], let him be anathema.”
I am sorry for such the long rant for just a simple question. I hope it helped.