I can't tell you one simple thing unfortunately as so many things work together
I stopped smoking - probably the biggest contributor to better health
I got my bloods checked and found very low vitamin d and b12 so I got that sorted (long process with a superdosing schedule of vitamin d supervised by doctor)
I got into a good sleep routine, again, long process which the vitamins helped with. I banned naps and lie ins for myself and keep to a set bedtime and get up time.
I lost 70lbs in weight
Exercise a minimum of twice a week
I changed my diet for the sake of my cholesterol. I avoid saturated fat, have plenty of good fats, moderate to high protein and plenty of fibre.
The sleep, exercise and food all contributed to the weight loss, which in turn again has helped my sleep... etc. Stopping smoking made it easier to exercise and sleep also. Lots of fibre especially soluble fibre also helps sleep.
I stopped drinking, never had a drink problem but because even one glass of wine makes me headachey and sluggish the next day and I just can't be arsed with making myself feel shit. I want to feel good (and I do now)
This has been a long process of about 6 years changing one thing at a time and sticking with it. I feel amazing now in comparison and much happier with my health.