Rickie Lambert scored a second-half hat-trick as leaders Southampton notched their 20th successive home win.
Lambert opened the scoring with an emphatic header just after half-time and added two more from penalties.
Veteran Brighton defender Mauricio Taricco was sent off for disputing the first penalty and thereafter the visitors fell apart.
Fouls by Lewis Dunk and Inigo Calderon, the latter on Lambert himself, helped him take his season's tally to 14.
The win, which maintains Southampton's five-point cushion over West Ham at the top of the Championship, had not looked as if it would be easy to come by in a first half that saw resolute defending from the visitors.
But Saints found a way through four minutes after the interval when right-back Frazer Richardson swung a deep cross to the far post and striker Lambert powered a header past Steve Harper.
It got worse for the visitors when referee Peter Walton pointed to the spot in the 57th minute after defender Dunk brought down Jose Fonte.
Dunk was booked for the foul but Taricco's protests brought the 38-year-old Argentine assistant manager a straight red card.
Lambert converted, before a fingertip save by Harper denied him a third. But the respite was short-lived as he completed his treble in the 69th minute, again from the spot, after he was fouled by Calderon.
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