There are a variety of problems associated with an idle mind. A person may become extremely analytical, excessively worried, or more easily overcome by temptation. In addition to all of this, a lack of productivity usually leads to a lack of profit as well. God warns us of the dangers of idleness many times in the bible. A couple of examples are listed below.
In all labor there is profit, but idle chatter leads only to poverty. Proverbs 14:23
Because of laziness the building decays, and through idleness of hand the house leaks. Ecclesiastes 10:18
Laziness casts one into a deep sleep, and an idle person will suffer hunger. Proverbs 19:15
Look, this was the iniquity of your sister Sodom: She and her daughter had pride, fullness of food, and abundance of idleness; neither did she strengthen the hand of the poor and needy. And they were haughty and committed abomination before Me; therefore I took them away as I saw fit. Ezekiel 16:49-50
The sleep of a laboring man is sweet, whether he eats little or much; But the abundance of the rich will not permit him to sleep. Ecclesiastes 5:12