Lately my brake lights keep coming on but I've checked the fluid and the levels are fine. I don't hear any grinding or shrill noises that would indicate that the breaks have worn down to the pads. Any clues as to why they would turn on intermittently?

1996 Buick Regal 3.8L 4dr Custom

When does the light come on? While applying the brake, holding foot on the brake at a light, or just driving down the road?

When does the light come on? While applying the brake, holding foot on the brake at a light, or just driving down the road?

it usually turns on when i'm applying the brake and switching gears usually backing up.