That's an impressive 10k time, Boobz - well done! I've recently gone sub-50 for the first time (convinced my stone weight loss has played a big part in that) and 45 mins is the next goal.
Sleep - the problem with an inbuilt "happy" weight, I think, is that it's very easy to convince yourself that you're at your "natural" weight when in fact, with a modern Western diet, there's not much "natural" about it at all.
Our diets have changed so much over the past few decades that people have become progressively fatter, convinced themselves that this is normal and healthy, and even convinced themselves that this is "natural." Backed up by the fact that we are surrounded by bigger and bigger people who also think that their weight is "natural", so a size 12 or 14 doesn't even look big any more.
For example, when I was 10st 10ish and a size 12, I was convinced that was my natural weight. I was super-fit at that size (ran a fell marathon) and struggled to get below it. I was utterly and totally convinced that was my "natural" and "happy" size, and maintained that weight for around a decade.
However, last summer I realised that, at a BMI of 24 and with a rather round face, stumpy legs and FF-cup boobs, I was on the larger side and perhaps my fitness levels would improve if I lost weight.
It was then that I started giving much more thought to what a natural weight is.
What I came to realise is that when most people talk about their natural weight, they talk about the weight they are when they eat what is considered "normal" now - ie maybe toast or cereal for breakfast, sandwich for lunch followed by a sugary yoghurt, ordinary sized dinner of pasta/curry/stir fry, the odd glass of wine, the odd chocolate bar. It's misleading to talk about "natural" weight when you eat things that even our great-grandparents, never mind our distant ancestors, wouldn't even recognise.
A true measure of natural weight would be to see what people weigh when they eat wholesome, natural foods and don't think that wine/chocolate/white bread/ chips/ beer/ pizza/ etc etc is part of a normal diet. People say they "only" eat chocolate twice a week or whatever, "only" have white bread occasionally, "only" have the odd takeaway - yes, these things are nice to have, but it's important to realise that this isn't what our bodies were designed to eat.
It was only once I stopped kidding myself about what a "normal" diet was that I shed the pounds, and now I'm maintaining at around 9st 8lbs. Yes, I eat chocolate occasionally and will have a couple of glasses of wine tonight. But now I realise that it's habits like this that can escalate and creep up, until before you know it you've gone from healthy to overweight, to obese, while still thinking what you're doing is "happy" and "normal."
Apologies for the long post.