I'm also on Lamictal, same dose as you (400). My pdoc considers it a high dose and that anything more does not have much of a benefit (if any benefit at all). I was on 600 mg of Seroquel too, but I was also feeling zombied out and so tired, although it helped significantly with mixed mania. After a period of time, I actually noticed my mood was better on 400 than 600, since there was less side effects.

You can think about the decision of cutting down on the Lamictal, but talk to your pdoc and it's strongly recommended to do so under supervision, since Lamictal has a lot of serious side effects and withdrawal symptoms without monitoring. I would never mess with tapering down on Lamictal on my own. You might want to consider that if the 400 mg of Lamictal is benefiting you, then maybe you want to avoid cutting down on it, if it's not causing any harm? 400 mg of Lamictal gives me no side effects, so I stay on that dose.

The Seroquel on the other hand, the less I need, the better due to the side effects. I've been guilty of cutting down Seroquel myself. Luckily, my pdoc agrees with adjusting the Seroquel as needed. I try to stay at 400 mg, which is a happy medium and only reserve 600 + for severe mania/mixed mania/psychosis. She told me anything lower than 300 mg is more of a sleep medication than mood stabilizer, but since you mentioned you use it primarily for sleep....then it's something to consider and talk about next time with your pdoc.

You can maybe try making just a slight change (with your pdoc's permission), and if there's any warning signs at all, catch it early enough to go back to your previous dose. Just throwing things out there! Bottom line: talk to your pdoc about your thoughts on this, since it's well worth discussing to make decisions.