Your 40s aren't about doing more. They're about signaling smarter. This is the decade where: Muscle mass naturally declines Insulin sensitivity shifts Cortisol becomes louder Sleep changes Insulin sensitivity in long lived growth hormone releasing hormone knockout mice Aging Muscle Insulin Resistance and the Inflamed Microvasculature: Fire from Within APPL1 Potentiates Insulin Sensitivity by Facilitating the Binding of IRS1 2 to the Insulin Receptor: Cell Reports