Similarities
As a single girl, Molly hasn't actually ever grown accustomed to sleeping alone. Not with humans, though. She got Toby the day after she moved to London to start her training at Bart's, and the faithful cat has been by her side ever since then. Especially at night.
Toby is a cat through and through, and if a cat says something is its bed, then that something is its bed. Unfortunately for Molly (or in retrospect, fortunately for her), the cat seemed to think that the big bed inside Mummy's room is his bed, and that Mummy only sleeps there to guard him as he sleeps. Or for body warmth. Or cuddle value.
And so, for most of Molly's single adult life, she has had her own furry bedmate, and she almost never gets a night free from the gentle purring or the tender kneading of her pet. Toby has this habit of sticking to her throughout the night, even as she moves from side to side, up to down, over and under. She's never minded, though. She thinks it's sweet to have the cat she loved so much constantly wanting to be in contact with her.
When she moved into 221B after her marriage to world-famous affection-phobic Sherlock Holmes, of course she has her adopted child in tow, purring in her arms as her husband and his best friend lug her suitcases up to the flat. Sherlock was surprisingly welcoming to the feline, even engaging it in conversations and letting it settle on his lap or on his chest during his trips to his mind palace. He however drew the line on the sleeping arrangements, refusing adamantly to share his bed and his wife at night. Molly could still remember Sherlock's face when he first came home to their bedroom from work and saw Toby napping happily on his side of the bed beside her. He had narrowed his eyes and wordlessly picked the cat up, marched to the sitting room and placing it on the couch, before walking back into the bedroom and slamming the door. Toby had ended up scratching and meowing through the closed door separating him from his mummy (and his daddy now, by law), and Molly remembers herself staring heartbroken at where the sound was coming from. Jealous, Sherlock had distracted her with some spirited and sensual lovemaking that night.
It's funny, though, that not much has changed to her sleeping experience when Sherlock replaced Toby as official sleeping companion. The two were remarkably similar in that aspect, as Molly would continually be amused by, that is, Sherlock is a surprisingly clingy cuddler in bed. The same thing: whether she's on her side facing him or away from him, whether she's on her back or on her stomach, whether she's half-propped up by pillows or flat like a corpse, it never matters. The consulting detective would be all over her, never a distance more than an inch between them. His hair in her face, his face against her neck, his hands everywhere and his legs never not tangled with hers or draped possessively over her thighs. The purrs, oh the purrs! Sherlock purred like crazy, a deep, sexy vibrating tone from within his throat, the waves transferring to her skin and enveloping her in shivers even in her sleep. It was almost like Toby had grown into more than six feet and was using it to his advantage.
They were so similar that the first few days of their marriage, she had accidentally called him 'Toby' in her sleep more than once, to which the detective always responded by either waking her up and then sulking until she cooed and sweet-talked him back to sleep, or proving that he definitely wasn't a cat by, well, doing something to her that she would never let a cat, or any animal, or any other person besides him, do.
As for Toby, well, he seemed to have accepted that Daddy needed Mummy to his own while they slept, and started enjoying the large couch as his very own kitty bed. And now that he's old (in cat terms, of course), he also enjoys not having to leave his bed at all as the day starts, just watching the humans go about their merry way.
Note: Because Sherlock is a cat in a consulting detective's body and nobody could convince the author otherwise.