Its cold, its dark, and youve got zero motivation. Well, one trainer has good news for you: you only need to commit to seven minutes, three to four times a week. Not only could you lose weight and build muscle something thats essential for everyone over the age of 30 but it builds up bone density and targets belly fat, a common midlifer problem. Sounds do-able, right?

HIIT is nothing new you see people doing it in their local park every weekend. But Zana Morris, personal trainer and founder of The Clock gyms in London, has a unique take: we need to do it much harder and for a much shorter time. "You should be totally exhausted after six reps, Morris says, then you move on to the next exercise.

"The key is to think of it like sprint training: you wouldnt sprint for 45 minutes, you run all out for a few minutes. Its the same with HIIT when youre doing it right its about short, sharp bursts.

When I join Morris well-heeled, mainly middle-aged clients at her luxe Marleybone gym for a month in December, I do a different circuit on each visit, either legs and bum, shoulders and arms or back and chest, and am out the door in under 10 minutes. Its a get in, get on, get the job done approach, Morris laughs.

Her approach, which she has been honing for more than 20 years, is backed up by a plethora of research, including a recent study in The American College of Sports Medicine (ACSM) Journal which found that seven minutes was enough to get the fitness benefits. You really can see a difference fast when you do seven minute really intense sessions, Morris says. On average, when you team it with the right nutrition, we see clients lose around 6-7lbs of fat and gain 2-3lbs of muscle.

The reason, Morris explains, is down to our hormones, particularly insulin levels. Any weight around our middle is insulin related, she says. Put simply, insulin, the hormone that regulates the levels of glucose in the blood, can cause weight gain when the cells absorb too much glucose or blood sugar and convert it into fat. Not only can it make you fatter, but in a catch-22 situation, increased body weight can also lead to higher insulin levels. Sleep affects your insulin levels, as does eating carb-heavy or sugary foods and stress all midlifer concerns.

But micro HIIT sessions can reverse that a study published in the journal Frontiers found that a ten-week HIIT training programme in sedentary adult women at risk for type-2 diabetes had positive effects on their insulin levels, while a separate Brazilian study confirmed the same thing, looking at sleep-deprived men and the effect that HIIT had on their insulin levels.

Theres an added benefit to performing HIIT, according to Pamela Peeke, assistant professor of medicine at the University of Maryland and Equinox Health Advisory Board member: The healthy stress your body undergoes during HIIT triggers autophagy, which rids your body of cellular debris and stimulates the production of stem cells, the primary regenerative cells in the body. The more stem cells you have, the better you are able to induce super autophagyits a cycle. Think of it as a spring-clean for your cells.

Peeke recommends HIIT training three times per week plus find every opportunity to add one, two, three minutes of HIIT to your day." Such as doing as many squats as you can in a minute while you wait for the kettle to boil or racing for the bus at a full-out pelt.

But for added benefit, as Morris has found with herself (she has roughly the same body composition now shes in her mid-forties as she did as a 20-year-old) and her clients, is to add in weights. After the age of thirty, we lose between 3-5pcof muscle per decade in a process called sarcopenia (most men will lose about 30pcof their muscle mass during their lifetimes). Its problematic because it not only leads to diminished strength as we get older, but makes us more prone to breakages, according to a study from the American Society for Bone and Mineral Research.

More pressingly, the amount of muscle you have has affects on your weight. Its estimated that 1lb of muscle burns about 50-100 calories per day, Morris explains. So, by the time youre 40, if youve already lost around 5lbs of muscle due to natural age-related wastage, you would need 500 calories less per day. But not many of us reduce our calorific intake in fact, we often increase it. The antidote is to try to rebuild that muscle.

When I train with Morris for four weeks over December, I build up my strength surprisingly quickly. In four weeks I am able to lift 110 kgs on the bench press machine, up from 80kgs at the start. In a month I gain 2.5 lbs of muscle taking me back to my twenty-something levels.

While having a trainer on hand to set your weights up is a luxury, the exercises are easy to replicate at home or in your own gym. You can do 60 squats in a minute, or as many as you can do, or swimming sprints in your pool. You can do it with almost any exercise, Morris says. But dont in the zeal of January restarts think that more is better. In fact, going over the 30-minute mark has negative effects on our ability to build muscle because we start to produce cortisol, which can lead to muscle atrophy. Morris shudders when she thinks of midlifers doing marathons and triathlons (I darent tell her Im one of them).

At the same time nutritionist Mackenzie Dumas looks at my diet. She points out that theres little point training if Im going to continue with my nightly Maltesers/ half a bottle of wine habit. For the first 12 days Im on a high fat, low carbohydrate diet which comprises of a lot of avocado and eggs, and almost zero carbs. During this period, I lose eight pounds of fat, according to Dumas callipers. Then she moves me onto a more sustainable high protein, low carbohydrate diet, which is broadly what the surgeon Dr Andrew Jenkinson author of Why We Eat (Too Much), recently recommended in The Telegraph.

The month under Morris's guidance has been a huge re-education in fitness. Out goes the half-hearted Sunday morning boot campbootcamp in the park, replaced by seven-minute power sessions with my kettlebell.

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Get ready for micro HIIT: the seven-minute workout that could transform your body, and your life - Telegraph.co.uk