It’s the free levels of testosterone, that correlate to morning wood. Many older guys start to lose them because as they get older Shbg increases. Shbg then binds up all the free testosterone making it unavailable. A safer alternative would be to eat foods that help lower estrogen in the body. Testosterone boosters have a whole bunch of side effects like increase hair loss if your prone to it. Eat healthy, get plenty of exercise, and lower alcohol intake.