I feel less grumpy and less anxious, each time I sleep my full 8, but I'd be hesitant to equate that sensation with "happier". If I do that for an extended period of time, I feel bored, existential, "socially unproductive", and begin to get a creeping sense of being a rat in a ratrace.

If I sacrifice some of those 8 hours, I feel grumpy, moody, occasionally a bit depressed, but overall get to do more small day-to-day things of value that give me a genuine sense of fulfillment on top of that.