If you go to your game here and look at the Stats tab, you will see a different number of stars in the start and end column for the players. This indicates how tired they got during the game. How much they tire during the game is influenced by their Stamina stat.

Then there is a completely unrelated other way in which players get tired...

if you look at your individual players you will see an energy stat. This stat indicates how ehhh... tired they are, but not from the games, but only from training. This energy stat is influenced by the training intensity setting.

So to summarize: players get tired during a game, and they can get tired from training, but contrary to what you might expect the fact that a player is active during a game has no influence on his energy level.

Eddy B wrote: The energy level of my player, after training i mean, does have an impact on their efficiency during the game tho right ? Assuming that if they have a low level of energy, below 70 or something, they would not play at they full capacity, correct?

Exactly! Actually the energy level appears to have quite an impact. On top of that, training on High or Maximum intensity increases the chance of injuries. An injured player not only cannot play in a game, but often also misses one or two weeks of training.
This is why most of the GridIron veterans only train their players on Average intensity.
Personally I sometimes train my guys on Maximum intensity, but that is at most one time per season, and not even every season.