The current HIIT trend is not sustainable and in my professional opinion, should not be completed without a 48-hour rest interval. This is because HIIT is extremely taxing on the body and central nervous system, it can cause unrepairable / significant wear and tear to muscles and joints. Plus, if exercises are not carried out in the correct form there is a high risk of injury and posture can deteriorate.
If you are completing HIIT on a regular basis (4-5 times per week) as advised by a Personal Trainer or online coach, then you really need to reassess your health and wellbeing. The long-term detrimental effect of regular HIIT training can lead to an ineffective metabolic rate, irreversible damage to muscles, ligaments, and joints, serious posture issues and long term injuries.
The current trend of body weight HIIT circuits on a regular basis (often recommended at 4-5 times per week) if completed at maximal effort in order to make HIIT training effective, makes it impossible to maintain safe and effective form for your body.
What exercise should you be doing?
I would personally recommend a variety of resistance weight based training and cardiovascular exercise that will allow you to shape, tone and promote a lean body. The key is balance, and HIIT training can be integrated into a resistance workout, however, I would only recommend this twice a week for no longer than 20 minutes, and based on a sprint-based workout, rather than body weight. i.e. on a treadmill, bike, cross trainer, rather than body weight exercises.
A good example of this would be 10 rounds on a treadmill, 20 seconds all out sprint, maximal effort, followed by 60 secs recovery (where you do nothing at all and completely rest.)
On other days, I would recommend doing 30-60 minutes of LISS cardio, (Low Intensity Steady State) which is great for targeted fat burn, providing you work at 60-70% above your resting heart rate.
A good example of this would be a power walk on a treadmill or on a stepper - it's far less taxing on the body and the central nervous system, it can improve your cardiovascular fitness and aerobic endurance, there's far less wear and tear on the muscles, joints and ligaments, and it can help you shape and tone a lean body. Plus, anybody can do it.