I've heard of people saying Lamictal made them tired, but it's never made me tired at all. I do take my 100 mg Lamictal in the morning, not night. I definitely don't think it makes me tired. In fact, it is more activating for me. I have always considered Lamictal more of an antidepressant for me, than a moodstabilizer.
If you continue to have hypersomnia at night, consider asking your psychiatrist what you might do. Would he/she consider moving your Lamictal to the morning, like when I take it?
I suppose an outstanding mattress could definitely improve/increase your sleep, but to 2 or 3 pm? That sounds unlikely, unless you're going to sleep at 3 am or later or have some other issue contributing. What time do you take your Lamictal at night? Could taking it a bit earlier at night help? Would your psychiatrist allow that?