The Reds succumbed to a 1-0 defeat against Graham Potter’s men and Klopp believes his men looked mentally tired.
After mounting a 68-game unbeaten run at home, Klopp’s side have now lost back-to-back league games at Anfield.
Liverpool seemed to have rediscovered their good form after away wins against Tottenham Hotspur and West Ham United, but Steven Alzate’s second half goal condemned the champions to their fourth defeat of the season.
Liverpool’s chances of defending the league has further been dented as they currently sit fourth; seven points behind Manchester City, who also have a game in hand.
"You saw what went wrong, it's more about why? The only explanation now is that we are a fatigued team, mentally more or less. That leads to not the maximum freshness in your legs. Brighton deserved to win, they played well, they did well,” Klopp said after the game.
"We had our moments but they deserved to win. They are a good football side. We had to defend with a lot of running, there is no formation in the world where you are always in the right place. The best way to defend against a team like that is to keep the ball yourself but we couldn't do it.
"The only thing we can do is use the things that happen and learn from them. The solution is always the players. It has been a tough week and our efforts were not enough. City are flying and we have to find solutions."