Bottom line up front: The dark can be your friend.
Turning on lights will most likely cause the intruder(s) to run like rabbits. However you are then left to worry about who they were, what they wanted, and whether they will come back. I am a worrier by nature, and these unanswered questions would prevent me from sleeping well for a long time.
I found myself in this situation in the summer of 91. I chose to leave the lights off until I had the upper hand. Results: Intruder held at gunpoint, proned out on the ground facing the floor. Cops arrived about 1.5 minutes later. I now know his name, where he lived, and what he wanted (as evidenced by the other 40 burglaries he was charged with.)
I plan to write up the entire experience and post it one day. Here is the abbreviated version:
Time: 2:40 AM
Wife and I are asleep. I have been back from First Gulf War for ~2 weeks. I am young (1LT at the time) and in good shape.
The doorbell rings repeatedly. I wake up, figuring it is a drunk at the wrong apartment and look out peephole (nobody is there), but never turned on any lights. There was enough light seeping through the blinds from outside to see. I go back to bed.
I here a noise from the second bedroom, grab .357 Ruger GP100 and investigate. I can see intruder through tilted blinds working at window. I return to bedroom and tell wife, who grabs Colt Gov. model .380 and dials 911. We still have all the interior lights off.
I go back to watch the intruder from outside the door of the second bedroom. He is a caucasian male, jeans, long hair with no shirt, using fingernail clippers to try and pry weatherstripping from window to remove glass. He is going slow, so I return to the bedroom to put on some pants (we were young and had no kids, so we slept in the buff.)
Intruder eventually breaks and removes glass. (Only after glass breaks does 911 place serious priority on call in a big city.) He climbs into pitch dark room, silhouetted in the light from outside. As soon as he is completely in the room, I flip on the light, cock the revolver and yell "Freeze - Down on the floor!" Yes there was profanity that would make any DI proud, but I am keeping it High Road...
He looks completely stunned and drops to the floor. He looks up to me to talk, and I shout "Keep your eyes on the floor, you don't have my permission to look at anything here!" He then begs for permission to use the bathroom, as if anyone in my position would find his request reasonable. I reply that he should go in his pants, because if he moves I will shoot him in the head."
The adrenaline in the situation is intense. The urge to hit him with something was tremendous, but instead, I just continued to berate him. I found myself wishing I had not cocked the gun so I could hit him with it. You really can't understand the urge to attack if you are not in the situation. Even today, thinking about the situation makes my heart rate increase and I get a bit worked up.
The cops arrived about a minute later. My wife was told by the dispatcher that they were there, to check through the peep hole that cops were at our door, and to let the dispatcher know that the cops were there before letting them in. (I am still not 100% sure why they did that.)
Only one cop entered the apartment (I was later told by neighbors that five police cars arrived,) and I only saw one other cop outside. The cop came in, turned his back on me (yes - I am still suprised by that 17 years later) and stepped on the bad guys neck. I turned the Ruger away from the cop and said "I'm going to put my gun away now." The cop said "OK." with his attention still on the bad guy and his back towards me. I went to the kitchen, and put the Ruger on top of the refrigerator.
Cops take bad guy to jail, and I never have to wonder who he was, what he wanted, or if he would come back.
Don't let this post hijack the thread. I will try and write up the experience in detail this weekend, with focus on the emotional, adrenaline charged aspects of facing an intruder, the thought process that went into the shoot - don't shoot decision as I waited for the intruder to break in, as well as the trial and punishment.
In my case, keeping the lights off gave me the upper hand. Your situation will be different.