Hi! Plenty of others have great suggestions for sleeping comfy and for addressing the sciatica (I would add that a prenatal yoga class would also be beneficial if you are not already doing so).
So... my question to you is, do you need to sleep on your left side?
My recollection is that it is suggested that one may feel more comfortable at night sleeping on the left side because the stomach tilts down and to the left - thus, when on your left side, any acids in your stomach are more likely to stay in there and not cause you heartburn.
If this is the reason, or if you have another reason to sleep on your left side then by all means find a comfy way to sleep. If you don't really have a reason to sleep on your left side, then don't bother. It is the same kind of thing as with laying on your back... for some people they are fine with that, others are built differently and will need to only be on their back for short periods later in pregnancy.
Whatever it is, always ask yourself why - if you know why something is recommended you will better understand how it applies to you, and you will also be more likely to be vigilant about it if it does apply (plus, if you stop doing it and have some stomach upset, you will better know where it came from)
- M.