As for dieting, I had a limited success with it - managed to visibly drop weight in few months, though I regained it over the year after I dropped the diet regime. Problem is, the diet was moderately distracting on a daily basis. The bigger problem, however, is that the vastly reduced variety of meals I consumed made me dislike some of them, and make others feel bland - so at this point, I can't see myself going back on the same diet, simply because I'm repulsed by the idea of eating those same foods again.
And back to the big point - as far as I checked, neither my weight, nor diet, nor physical activity seem to have had any noticeable correlation with my sleep patterns, mood, energy levels or ability to focus. I've never felt any positive effect within days or weeks of improvement in physical activity or dieting - nothing above noise floor, and especially nothing I could clearly attribute to lifestyle change.
An untrained individual(not morbidly obese) who incorporates the following 3 minute workout in their morning routine will feel the changes already on day 2:
Hold a plank, for as long as you can manage
30 second break
Push ups, as many as you can do
30 second break
Again do push ups, as many as you can do
The extremely short body weight workout I described will still create a noticeable improvement and can be improved upon on when the individual feels ready. What I described will be a good first step and is hardly akin to studying nuclear physics. You don’t have to be an Olympic level athlete to reap the rewards of daily exercise.
In general, an improvement in diet and physical activity, even small changes, has an almost immediate positive effect. Clarity of mind, improved self confidence, better sleep…