Sloth, in a sense, is laziness but it is not plain laziness. Catholic Reference Net defines sloth as
Sluggishness of soul or boredom because of the exertion necessary for the performance of a good work. The good work may be a corporal task, such as walking; or a mental exercise, such as writing; or a spiritual duty, such as prayer. Implicit in sloth is the unwillingness to exert oneself in the performance of duty because of the sacrifice and the effort required. As a sin, it is not to be confused with mere sadness over the inconvenience involved in fulfilling one’s obligations, nor with the indeliberate feelings of repugnance when faced with unpleasant work. It becomes sinful when the reluctance is allowed to influence the will and, as a result, what should have been done is either left undone or performed less well than a person is responsible for doing. Sloth may also mean a repugnance to divine inspiration or the friendship of God due to the self-sacrifice and labor needed to co-operate with actual grace or to remain in the state of grace. This kind of laziness is directly opposed to the love of god and is one of the main reasons why some people, perhaps after years of virtuous living, give up on the pursuit of holiness or even become estranged from God. (Etym. Middle-English slowthe, slow.)
Now, there is nothing wrong with taking a day off or some vacation time. God himself rested on the seventh day, appreciating the work he had done in the six days of creation.
Laziness becomes a problem when it keeps us from doing good works and taking care of ourselves. As Jesus calls us to help those in need around us, failure to do so when we are able offends God and thus is a sin. This does not mean that it is always a sin to not help those in need. Sometimes we are in need ourselves and do not have the means to help others. God does not ask more of us than we have to give (realizing that we should not put our own pleasure before the needs of others). Our failure to help others becomes a sin when we could help but decide not to because it would be too much “work.”
What about taking care of ourselves? The definition above from Catholic Reference Net includes not taking a walk. God has given us the gift of life and we show we value that gift by taking care of ourselves with exercise and avoidance of gluttony. The definition above also includes “failure” to pray. Never taking time to pray is like saying it doesn’t really matter. Are we all experts in prayer and getting the most out of it? No, sometimes we need the help of our spiritual director. But a decision not to pray because it isn’t worth the effort offends God and hence is an occasion of sin.
Lastly, sitting down and watching TV or reading is not necessarily laziness. First, it can be rest. It can also be educational. The place to start if we want to become better people is to focus on patterns of behavior (we should desire to improve all our behavior but start with the bad behavior that we do often). This is true of all of the Seven Deadly Sins. Sometimes we even find, that if we begin with sins that we commit often, that as we work to change one behavior others may get better at the same time because they may have the same cause.
Now, “Go, and from now on do not sin any more” (John 8:11).
P.S. Is it ironic that “sloth” is the last of the seven deadly sins covered here?