The app called Dimmer will take you from 10 to 0 (10 is as low as you can go with the stock brightness control). However, I later found an app called Screen Filter which works even better - you can make the screen even darker, I found 0 to still be very bright in a dark room.
Even better, you can set up Screen Filter so it dims the screen a small amount all the time to save some battery life.
Best phones I've had in 18 years of owning a cell phone ...................
1994 Motorola MicroTAC First "compact" cell phone, ever. My first cell phone, ever.
2007 HTC Advantage First phone with a 5" screen, ever. Had a stylus, too.
2012 Samsung Note Brilliant 5.3" screen. Useable form factor. Decent UI. Stylus. Best phone, ever.