Why Do You Feel More Tired After 40?

Testosterone declines in men from roughly age 30 at about 1% per year — by 40–50, this is often clinically meaningful, contributing to fatigue, reduced muscle mass, and lower motivation. In women, perimenopause (often starting in the early-to-mid 40s) brings oestrogen and progesterone fluctuations that disrupt sleep and affect energy and mood.

Thyroid function also becomes more variable with age. Subclinical hypothyroidism (mildly low thyroid function, often undiagnosed) becomes more common and causes fatigue, weight changes, and cognitive slowing. A full thyroid panel (TSH, free T4, free T3) is worth requesting if fatigue persists.

Deep slow-wave sleep decreases naturally with age — adults over 40 typically get significantly less deep sleep per night than they did in their 20s, even with the same total sleep time. This reduces sleep's restorative value and is why many people in their 40s report waking less refreshed than they used to.

Sleep apnoea prevalence increases with age (and weight gain, which is common in this period). Undiagnosed sleep apnoea is a significant and fixable cause of mid-life fatigue.

Resistance training becomes increasingly important: it preserves muscle mass (which drives metabolic rate), maintains testosterone sensitivity, and improves sleep quality. Even two sessions per week produce meaningful benefits within 8–12 weeks.

Nutritional needs shift: protein requirements increase (to maintain muscle mass with reduced anabolic signalling), vitamin D and B12 supplementation become more important (absorption declines), and overall caloric needs may decrease slightly while micronutrient needs remain.