Wayne Andrews split the teams with a fortuitous effort from the right touchline to secure a battling win for improving Colchester United.
This scrappy game looked a 0-0 stalemate all the way until Andrews came up trumps with his third goal of the season on 67 minutes.
It was a bolt out of the blue. From the right wing, Andrews delivered a cross-cum-shot which flew over keeper Ben Roberts and into the far corner of the net.
Brighton, who were fifth in the table before kick-off never got going. They were quickly closed down by Colchester defenders and their star striker Leon Knight rarely got a kick of the ball.
However, the Seagulls almost sneaked an equaliser in the 74th minute when Gary Hart's low shot from eight yards out hit the post.
But chances were few and far between in a game that lacked rhythm with more than 40 free-kicks conceded.
This was Brighton's second league defeat of the season, and after the match their manager Steve Coppell said: "If Wayne Andrews had meant to put the ball there then he'd be playing in the Premiership and not for Colchester United." Andrews has now scored three goals in five appearances for the U's since his summer move from Oldham.
And Andrews admitted: "The lads said that it looked a spectacular goal, but I didn't actually see the ball go in. It was not really meant." Colchester lost their first three league games of the season but they are now unbeaten in four. After the Andrews goal, they had a one chance to add a second when Kem Izzet lifted a shot over the bar from Karl Duguid's cross.
The U's manager Phil Parkinson said: "We didn't get much luck in our first three games so we are due some."