The Turin giants, who are also competing in the Europa League, were beaten for the second time this season in Serie A after losing 2-0 at the San Paolo stadium.
"We were off colour," Conte said to Gazzetta dello Sport. "We are playing so many games right now and always with the same players due to injuries.
"Most of my players are tired as they haven't had a chance to rest."
Juve struggled without Serie A's top scorer Carlos Tevez, who missed the encounter in Naples through suspension. The Argentinian has netted 18 league goals this season.
"This is the type of game that brings out the best in Tevez," Conte said. "We missed him but I thought those that played did a good job."
Conte admitted third-placed Napoli, who are 17 points adrift of Juve, were the better side on the night.
"We didn't play well in the first half and we have to give credit to Napoli," he said. "They deserved to win.
"Once in a while it's good to a get a football lesson. However, the team that gave us this lesson has been built to win.
"Napoli have invested 100 million euros in their team and they have a very good squad.
"We are ahead of them in the standings because we have been more consistent throughout the season."
Juve are 11 points clear of nearest rivals Roma at the top of the standings with seven league matches remaining, although the Eternal City giants have a game in hand.
The Turin outfit travel to Lyon in Thursday's Europa League quarter-final first leg before hosting Livorno next Monday in Serie A.