Four fire engines and 21 firefighters and officers were called to a fire at a house in Helix Road in Brixton.
Half of the roof of the property was damaged by fire. One man was treated at the scene by firefighters.
Station manager Chris Payton, who was at the scene, said: "Crews did a great job utilising the thermal image camera to locate and then subsequently extinguish the fire."
The Brigade was called at 0418 and the fire was under control at 0545. Fire crews from Brixton, Clapham and West Norwood fire stations attended the scene.
The cause of the fire is believed to be due to an insulated halogen downlight igniting timbers in the loft.