Every night, Karen Sparrow rests her head on her pillow, pulls the covers around her and sucks her thumb.

But Karen is not a toddler trying to soothe herself to sleep. She is a grown woman of 38, with a job as a data analyst and a family — and this bad habit, left over from childhood, causes serious disruption to her life.

While concentrating at her desk, Karen’s thumb creeps into her mouth. Driving in her car, she unthinkingly takes one hand off the wheel to suck her thumb. Once, she even started doing it in a job interview.

According to a recent survey, 12 per cent of adults continue sucking the childhood habit of sucking their thumb

‘I was in front of a panel when everyone started staring at me and I thought: “Did I answer a question wrong?” Then I realised: I was sucking my thumb,’ says Karen, from North London.

‘I apologised and they all started laughing, saying that they’d never seen anything like it before. I was so embarrassed, but I did get the job — I think it made a good talking point!

The habit is so ingrained that, when Karen dislocated her shoulder seven years ago and had to wear a sling — preventing her from sucking her left thumb as usual — she ended up sucking the right thumb instead.

‘I couldn’t not suck my thumb,’ she says. ‘I find it so comforting and relaxing. I do it while I’m watching TV and in bed where it helps me to go to sleep. The only time I feel self-conscious is at work, because I know it looks so childish.’ Clinical psychologist Helen Nightingale says adult thumb-sucking happens automatically and can have a familial cause. ‘It’s often passed down in families,’ she says. ‘It’s highly self- soothing and, as such, it is very resistant to change.’

Her mother used to paint Karen’s nails with varnish so the bitter taste would put her off. ‘But it never worked,’ she says. ‘And now, I don’t want to stop.’

The only time Karen thinks twice about her habit is when she’s with her 11-year-old daughter, Ashley, who cries out with embarrassment when she sees her mother inadvertently sucking her thumb in public.

‘If it happens when we’re out shopping, she’ll give my hand a light slap and tell me to stop,’ says Karen. ‘I know she finds it ridiculous because I’m a grown woman, and I suppose it is.’

Ridiculous it may be, but according to a recent survey, 12 per cent of adults continue this childhood habit.

No one knows exactly why the thumb is sucked, but it is likely to do with ease of access as a foetus and baby

Its persistence is partly due to the fact that sucking is the first reflex we learn; it is a survival instinct developed in the womb that becomes the sucking reflex responsible for breastfeeding.

No one knows exactly why the thumb is sucked, but it is likely to do with ease of access as a foetus and baby.

Sandra Trebinski of the Hypnotherapy Centre, in Windsor, Berkshire, sees at least 200 or 300 adult thumb-suckers a month — and believes the habit can be down to attention-seeking as well as its ability to provide emotional comfort in times of stress.

‘While most children drop the habit naturally, those who continue will often receive parental criticism which, while negative, is still attention and works to get them noticed,’ says Sandra.

According to a recent survey, 12 per cent of adults continue this childhood habit

‘The longer that the habit continues, the more it becomes ingrained in the subconscious and becomes an anchor activity that induces a relaxed state of mind.’

However, those who come into Sandra’s clinic are often desperate to break the habit.

‘Many say how essential the thumb-sucking has been as a stress reliever, whether at work or through exams, but they have finally reached a tipping point and want to stop,’ says Sandra.

‘Often it’s to do with their teeth — the sucking can damage the shape of the jaw and position of the teeth, causing them to protrude.’

And if clients are about to get braces to rectify the damage, they don’t want to undo all the work by thumb-sucking.

‘Or perhaps they’ve changed job or got a new partner. There’s a fair bit of secrecy and shame and fear of being found out, which can be reason enough to stop.’

Sandra’s sessions, which are 90 minutes and cost £95, involve putting her clients into a hypnotic trance then suggesting they leave the habit behind.

Usually one session is enough to break the habit that has lasted since childhood

‘Thumb-suckers by nature tend to go into a trance-like state when they’re sucking so they are often more open to hypnotherapy,’ she explains.

Usually one session is enough to break the habit that has lasted since childhood.

Natalie Rogers, 33, who is training in child and adolescent mental health and wellbeing, has no intention of seeking treatment for her thumb-sucking, even if it means having to hide it from potential boyfriends.

Natalie, who lives in Southgate, North London, says: ‘I remember at 16 sitting in my then boyfriend’s house and his mum scolding me, telling me that it wasn’t appropriate for a girl of my age.

‘As I’ve got older, I do it mostly at night when I have trouble falling asleep.

‘When I start dating someone, I have to do it secretly, though sometimes I’ve done it without thinking and have had to quickly take it out.’

Anna Reay, 35, has also found her childish habit disruptive to her love life.

The singing coach from Newcastle upon Tyne confesses that after a less than successful night out with a boyfriend, she went home early just so she could suck her thumb in bed.

‘It has been such an important part of my life for so long and I’ll do it everywhere,’ she says.

Usually one session is enough to break the habit that has lasted since childhood

Her fiance Stuart, 38, who runs a joinery business, rolls his eyes and tells her to stop, but she says the only reason she would quit is to protect her teeth.

‘I had braces for years when I was younger and I now wear a retainer behind my front and lower teeth, but I think they’ve definitely moved forwards,’ says Anna.

Orthodontist Dr Neil Counihan set up Metamorphosis Orthodontics, the UK’s first thumb-sucking clinic, in 2008.

The clinic, in South-West London, targets children between eight and 18 as well as adults.

‘Persistent thumb-sucking can reshape the mouth,’ he says. ‘Before the age of six, it’s fine as your adult teeth haven’t come in, but once they do the problems can start.’

Like any habit, Dr Counihan says it takes a conscious effort to break it.

‘You can try physical barriers such as thumb guards, but ultimately you have to want to stop,’ he says.

Karen, luckily, hasn’t had any problems with her teeth, though her favourite thumb for sucking, she observes, is inexplicably slightly shorter than the other one!

‘As habits go, it’s harmless and not a bad one, is it?’ says Karen, as she — you guessed it — starts to suck her thumb.