My apartment is pretty small, dark and airless so I have to leave the lights on in nearly every room to keep them lit sufficiently to work in them. It's not that there are no windows, but rather that the surrounding buildings block out a lot of the light most of the time. The only time they don't block out the light is around 4:30-5:30 am when the sun shines brightly just behind my husband's and my sleeping heads. Anyone who doesn't believe in daylight savings time should have to put up with all that candle power shining on them at an ungodly hour. It'll change your mind.
Because my apartment is so dark, my habit up until a month ago had been to leave the lights on in both the living room in which I actually teach and the kitchen which is directly behind me. The students walk through the kitchen to get in and out (floor plan here to clarify) and I felt it might make them uncomfortable if they looked out onto the yawning dark behind me as we spoke. Well, it's not a big space so it's more of a tiny yawn, but you may see my point.
There was also a bit of a logistical problem in terms of smoothly welcoming the student in (with the light on, of course), pouring tea or coffee as the student is seated in the adjacent room, and then having to break eye contact and interrupt preliminary chatting with the student to walk over to the entrance and turn off the light before carrying the tea into the room. Also, when the lesson was over, I'd have to again break eye contact and go turn the light back on before the student entered the kitchen. In order to avoid the feeling of being in a dark apartment and this awkward set-up, I just left the light on all the time and figured this was a sacrifice I'd have to make for doing business in my apartment just like I have to be sure to use heat in the winter and air conditioning in the summer to be sure students are comfortable.
About a month ago, I decided that the environmental impact of running a light in my kitchen when no one was in there for between 40-50 hours a month was not worth the dubious benefits and I started to turn it off just after students sat down and on before they left. The situation is still a bit awkward for me, but the need to turn it on actually helps give me and excuse to get up at the end of the lesson time and head for the kitchen (to get the light) ASAP so the student can put her shoes on. This tends to have the highly desirable effect of getting them to pack up and hit the road a bit sooner and not steal quite as many extra minutes from me at the end of the lesson.
This month marked the first contiguous block of time where I could see the measure of my efforts in regards to the lights. Though my kitchen lights are fluorescent and I only use half of the strip (one tube instead of two), my electric bill went down by between 500-600 yen ($4.60-$5.50). This is a relatively insignificant amount of money saved, but it does show rather clearly that there was an appreciable amount of energy wasted in the use of just one fluorescent tube (albeit for quite a lot of total hours). Every time I forget to turn off a light after going to another room, I'll be keeping this little example in mind to motivate myself to go back out and turn it off.