Back before I was first diagnosed, I noticed how much better I felt in the morning after a good night sleep and how I got worse as the day went on. Of course my body ultimately could not keep up with the advancing cancer, but it appears that my immune system was making an effort. Right before I started treatment, I took an trip to Europe that turned out to be a big mistake. I could not sleep on the long flight and ended up going over 24 hours without sleep. After losing that much sleep, I was incredibly weak and could only walk around 20 feet before sitting down. I returned right away, but needed wheelchair assistance the entire way back. Once I started getting normal sleep again I improved although I was still in a lot of pain. I started treatment soon after.
Here's an article on the subject: