Assuming they're coldblooded, as that seems to be your worry, they'd be diurnal. The other answers are pretty clear on that. However, you can still have them work at night or even in closed spaces for long periods of time since, if they're sapient, they would have at least rudimentary technology.
According to this article on the terrible about.com, reptile pets need both a proper temperature gradient and ultraviolet to be healthy.
Heat isn't a big problem: stoves, heaters, radiators, torches, all kinds of things will work. They might not have high requirements depending on the climate and how fast they lose heat of course. Using these, they might not really have to bask in the sunlight during morning hours (or they get around this by sleeping naked and having windows always facing east, perhaps skylights would facilitate this further).
Ultraviolet is harder to artificially produce without some modern technology - at least I'm not aware of low-tech solutions. The issue is that you need very high temperatures, upwards of thousands of degrees K, to get UV emissions. However, if I understand the pet stuff properly, it's necessary to get some UV exposure and also to get it at proper intervals so you don't mess up the day-night cycle. It doesn't seem to be as critical as heat. Exposure to sunlight during the day would do a good enough job I think. Plenty of windows would help.
So all in all, you don't really need to have them bask in the sunlight for hours and definitely not all day. They'll get enough of both with proper dwellings, some fire and windows.