When we think about the word "sin", it is easy to think about drinking, adultery, murder etc and feel that we are living a good Godly life.
We fail to see the big consequences of laziness. The work undone, the prayer never prayed, the gospel never shared, the help[ never extended, the light never shined - all of them are sinful and affects our life and the lives of people whom God has placed in our lives.
“Therefore to him that knoweth to do good, and doeth it not, to him it is sin.”
According to James 4:17, we sin by not doing the good thing we are supposed to do.
Our lives are time-bound. We have opportunities to store up treasures in heaven only during our limited time. Rest, spending good time with family and friends etc. are good and can bring glory to God. But being idle, doing nothing and purposefully wasting the time and resources God has given us is a sin.