Alongside their push for a top-four Premier League finish, Rafael Benitez's men are also competing in the FA Cup and UEFA Europa League.
After next week's international break, Chelsea will take in a run of six games in 16 days, including a two-legged Europa League quarter-final and the replay of their FA Cup last-eight tie with Manchester United.
While reluctant to make excuses for the Blues' inconsistent form this season, 27-year-old centre-half Cahill admits the congested fixture list is proving tough.
He told the Daily Telegraph: "People criticise you if you say you are tired, and we know we have to get on with it because the fixtures are there.
"But all I am saying, as a general fact, is that players do not go into every game feeling 100 per cent fresh, it is impossible.
"At a big club every single game is so important, but you might be going in 60-70 per cent fresh through no fault of your own.
"Everyone is trying to recover but you just don't have the days. It is not an excuse, but it is a fact."
Cahill, who joined Chelsea from Bolton in January 2012, has notched up 38 appearances for the club in all competitions so far this season and has missed just one Premier League game this year.
Despite cementing his place as a first-team regular under Benitez, the England international is taking nothing for granted with the likes of John Terry and David Luiz also competing for the centre-half berth.
He said: "The main thing is that I am playing in a lot of the big games, I am playing more regularly now, but we have got the luxury of four centre-backs now so we can chop and change.
"I think maybe I proved when I first came that I could play in the Champions League, because we went on and won it, and I played the games in there. Playing with England has helped me as well.
"You come here from a smaller club and people wonder, can he play here, can he do this? And you always feel like you are trying to prove yourself to people all the time. For me now it is just trying to prove that I can maintain a starting place at Chelsea."