Does Cycling Improve Your Mental Health?

If you're a regular cyclist you probably know that feeling when you haven't been out on the bike for a while. Bit more irritable, bit more restless perhaps? Or a lot more? With 1 in 4 people in the UK experiencing a mental health problem in the UK I've often wondered over the years about the impact of cycling on mental health, my own included.
So a little ride out on the MTB in my local Pennine hills and a few ramblings about some of the possible benefits of cycling. Do you ride for any of these reasons?
Happy cycling folks!