Low testosterone after 40? Start with how you live.

You're past forty. At the gym you do what you did ten years ago, but your muscles respond less. Your belly is growing even though you eat the same as before. And in the evening you have less appetite for everything: going out, sex, sometimes even an argument.

You tell yourself it's work, and that's probably part of it. But in many of the men I've worked with over the years, once they were past forty, a hormone nobody had ever talked to them about properly was also playing a part: testosterone.

What it really does for you

You know it as the hormone behind desire and muscle. It does far more. It supports your concentration and mood, keeps your bones strong, helps you make red blood cells and keep body fat down.

Why you notice it late

It falls with age, and it starts early: in your thirties. It goes down slowly, by about one per cent a year, and that's exactly why you get used to it. A little less energy each year, until less becomes your normal. It doesn't happen to everyone in the same way. Lifestyle makes a big difference.

What pushes it down

• Short nights. Testosterone peaks in the morning, after a proper night's sleep. Five hours instead of your usual seven or eight can lower it by 10 to 15 per cent.
• Your belly. Body fat contains an enzyme, aromatase, that turns testosterone into oestrogen. The bigger your waist, the more you convert.
• Alcohol. It makes the same enzyme work harder.
• Stress that never lets up. Cortisol kept high for a long time holds back your sex hormones. When your head is full of worries, desire is the first thing to go.
• Too much sitting, or too many miles. People who sit all day tend to have lower levels. And long hours of endurance work, like marathons or endless rides on the bike, bring it down.

What protects it

You can't do anything about your age. These five habits are up to you.

Get your hours in. Seven or eight, at regular times: going to bed at a different time every night makes your sleep lighter. Last coffee straight after lunch. As soon as you're up, get out into daylight, which resets your body clock. And in the evening, dim the lights and switch off your screens. Bright light at that hour pushes sleep back.

Train for strength. Lifting weights is what stimulates testosterone most. You need loads that challenge you and proper recovery between sessions: training hard without recovering works against you. If you have the choice, train in the late afternoon, when cortisol is already low and testosterone has a second peak.

Take inches off your waist. But no crash diets: eating too little lowers testosterone, and so does cutting out nearly all fat, because your body makes it from cholesterol. Eggs, extra virgin olive oil and meat have their place on your plate.

Keep alcohol for occasions. Dinner with friends can stay. What counts against you is the drink to unwind, every single evening.

Let the pressure out every day. Make sure work stress is done with before the evening. When you feel the tension rising, take five slow breaths: four seconds in, six out.

These habits aren't a cure. They're what your body has to work with every day. Start with sleep, which underpins all the others, and give it a few weeks. Then add strength training.