Actually this is not true. For some people Lamictal can cause drowsiness, or make them feel emotionally flat. For others it can be activating, and even cause trouble sleeping.

I am in the latter camp, as Lamictal is activating for me and gives me energy, so I have to take it in the morning. Some people get drowsy, relaxed or even dizzy from Lamictal and it is better for them to take it at night.

Common side effects of lamotrigine include:
Dizziness, sleepiness, tiredness... And also paradoxically: insomnia! Go figure.