How I feel after a session mostly. It's somewhat intangible and anecdotal, but when I have been over to Bavaria for driven boar, there's always been quite a few nights of heavy drinking but despite this I have always awoken fresh the next morning, I think because I always drink plenty of water during and before head hits the pillow, but I also suspect that it is because of their purity laws. I've made beer following it and I've never had a thick head after, even though I haven't drunk it in the quantities I did in Bavaria. I have had less heavy sessions over here, making sure I drink water in between but still wake up with a dreadful hangover. It's anecdotal like I said, but I suspect it is something to do with the sulphites