This is similar to my experience (iPhone 5S, 8.0.2), though often I don't even need to reboot. Open the Health app; no data; close it; do something else for 1-2 minutes; reopen - data is now visible.
I tried the leave it and come back and it didn't work for me, only a full power-off cycle. It does seem to be slightly different quirks across different user devices.