I've read that too. That the soft tissue around the joints fills with fluid while sleeping and it takes time to get back to normal. During this time it is supposedly easier to get injured.

I've worked around this a few ways. One, it takes 10 minutes to get the the gym. I also have to get up, brush my teeth, put clothes on, etc... Second, when I do any workout, I warm up. Walk on the treadmill, then some stretching, and finally warm up to the weight I'm going to use.

I've never had an issue, except now that I'm older it can take longer to get going :)