going back to work soon, how do I tell my boss that I can't work as late as "before-baby" period? I used to be very dedicated to my work, staying back to work till 11plus at mist days.. now I so tired (my lo doesn't sleep thru the night), and I fall asleep after my lo sleeps at 9-10plus..

Honesty is the best policy. You can be honest about it, but be prepared of the repercussion effects in terms of the impact on your career progression. Given the constraints on your job flexibility, it's not going to do good to your job stability in a difficult market environment as well. That said, I'll pick my family over my career anytime. Assuming that your boss is understanding and pro family, have a discussion on the possibility of changing your job profile such that it's likes time sensitive and allows you to plan ahead.