Sebastian takes care of himself pretty well. Because he needs his body to be a temple, essentially, so to make up for smoking and drinking habits he goes out of his way in other areas of his life to make sure he stays in good health.
This means having a bedtime.
If he's in London and able to get to a flat, he'll always be in bed somewhere between eleven and midnight, when possible. And no matter how well he sleeps, he'll be up again between six and seven in the morning.
If he's out, or with someone, or on a job, or doing other work he'll find a way to compensate, but very rarely sleeps in. He doesn't like giving his day away to sleep, he'd rather push through it for joy of a better nights sleep the next day.
☠ : Drinking/drunk headcanon
It takes a lot more for Sebastian to get properly slaughtered. So he'll only go out looking to get drunk if there's some really big reason. Whether that be something to celebrate or something he wants to forget.
His two main pubs are Filthy MacNastys on Amwell Street and The Crown (The Crown Moran Hotel) on Cricklewood Broadway. And that's what you'll find him mentioning most often, or where you'll tend to find him. Mostly he'll just go down for a few drinks, casual, out of habit. But if he's drowning his sorrows, the majority of people who know him down pub can tell and know to stay away.
His most favourite drinking buddy is dear old Shinwell 'Porky' Johnson. The majority of what they do is drink together. But he's probably one of the few good friends Sebastian really has because of that casual friendship.
Having daddy issues is a bit of a stereotype, isn't it? But it is the case. Frustratingly for Sebastian, it seems to be something which (every) Sherlock seems to pick on up when they met (meet). He doesn't know what it is about him that gives it away, but he's given up trying to figure it out.
Yes, his dad was a military man, so he kept a tight ship. Austen was abusive towards his mother, and from the time Sebastian turned four or five, he kept a firm had with him too. Though he absolutely resented his dad, by the time he was old enough to realise how wrong what was going on in that house was, he was shipped off to Eton and didn't see home quite as much.
Bad memories associated with this abuse and his mother baking in the kitchen are the reason he can no longer eat sweet food without feeling violently nauseous.
He maintains that he would have killed Austen himself, but the git died himself before Sebastian got the chance. And that's his biggest regret.
As egotistical as Sebastian appears in general day to day life, and especially when flirting with those lovely long-legged creatures known as women... he's pretty... generous in the bedroom. He's not the kind of self-obsessed cunt that stares at himself in the mirror the whole time. He likes to provide each willing woman with a unique experience because he enjoys taking them to the absolute edge and back again.
He will do everything in his power, every trick up his sleeve, just to give his partner the best orgasm, or general experience, of their life. And then just walk out the door never to be seen again.
He's like a sexual superhero.