Many studies say that hgh upregulates igf-1. intermittent fasting (8hour feeding window) boosts gh-levels. why than is if associated with lower igf-1?

Not so simple: The regulation of IGF1 is more complex than just GH levels. Diet/weight loss and inadequate nutrition is well known to lower IGF1. GH levels spike in the AM, but with age GH levels are low, and there is not much you can do to elevate them. Exercise, stress reduction, weight maintenance are all good, but not necessarily related to GH.