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  • Holly Leake

Why Does my Dog Snap if I Wake him? Understanding Startle Reflex

Updated: Sep 10, 2023

"Sleep makes anything look innocent until it wakes up." (Unknown)

Ever heard the saying, 'Let sleeping dogs lie?' You've probably heard of it but most have forgotten the reason for this warning. Although this saying wasn't originally intended to be literal, it's true you should never wake a sleeping dog. Why is this the case?

What happens when someone shakes you awake unexpectedly, when you are in a deep sleep? At first, you panic, because you are still living in that dream and your brain is trying to catch up and make sense of what's real and what's not. The deeper the sleep, the more startled we are when we are awoken.

Likewise, our dogs experience vivid dreams in their sleep. Although they need more sleep than us (14-16hrs) they do have a similar sleep cycle, including REM sleep (rapid eye movement), which is when your dog will dream the most. So what happens when you wake your dog? Well, your dog can be just as startled as you would be, especially if your dog was dreaming. While you may initially jump and then scold the person for waking you, your dog will likely communicate the only way they know how.

It is very common for dogs to lunge, snap or bite when they are woken unexpectedly. This behaviour is actually known as a 'startle reflex' or 'sleep startle' and it is an instinctual behaviour seen in many animals, including the Canine ancestor, the Grey Wolf. There are many potential dangers when living in the wild. In order to survive, many animals have to assume that whatever has disturbed them is a potential threat. When you think about it, it is very much a life and death situation. Even though domesticated dogs aren't wolves, they still have some instinctual behaviours, including this startle reflex.

The startle reflex can affect dogs of any breed, age or size, however their environment can make them more vulnerable to a severe startle reflex. Dogs bred and/ or kept in stressful environments, may struggle to feel safe when sleeping, making them more prone to startle. Dogs that have also not had adequate socialisation with people, such as street dogs and puppy farmed dogs, may also be particularly effected by startle reflex.

On the other hand, any pet dog can have a startle reflex. If your dog suffers with noise sensitivities, anxiety and/or chronic stress, this may predispose them to a startle reflex. Age or health may also be a factor, as dwindling hearing may cause your dog to easily startle if you touch or speak to them unexpectedly, while they are sleeping. Health conditions and pain can naturally cause a dog to feel more vulnerable, which could also contribute to a startle reflex.

Although some find this behaviour very alarming and refer to it as sleep aggression, I promise you that it is not aggression. When your dog snaps after being awoken, they aren't consciously choosing to attack, rather it's just a knee jerk reaction. This is why there is no body language or growling to warn you, like there is with typical aggression. Once your dog recognises you and where they are, they immediately settle. Unfortunately, for some dogs it's too late and the damage is done, especially when children are involved.

Veterinary Centres of America (VCA) (2021), estimates that, "60% of dog bites in children occur when the child wakes or attempts to wake a sleeping dog". In the UK, 75% of dog bites involve children, with a bite to the face or head. So clearly this demonstrates that any family dog can bite due to startle reflex, and that children are the most common victims. Simply touching the sleeping dog, talking near their face or kissing them on their head can be enough to startle a sleeping dog. This makes sense since most dog bites involving children are inflicted on the face.

We would all love to believe that our dog would never bite us or anyone else. Unfortunately, studies show that most bites are inflicted by their own family dog or a dog they know. You probably can't fathom your gentle dog biting you or your family. It would be completely out of character for them, but sadly this is often what families say after their dog has seriously bitten them or a family member, and its seen time and time again.

One experience of this, involved an almost fatal attack on 3-year-old Charlie Stirton in Australia in 2018. He climbed on his senior sleeping dog and the dog startled and bit Charlie, fracturing his skull and breaking his nose in one bite. Fortunately, Charlie survived, but his dog was not so lucky. This tragic event was a shock to the parents who emphasised how friendly and gentle their dog was with the children before the incident.

"No matter how familiar the dog is, and even if it is understood to be a friendly dog - something I hear all the time - if that dog is even accidentally woken up with a sudden start, or accidentally poked in the eye, or pulled on the ear, or given a fright, their natural instinct is to snap." (Dr O'Mahony 2014)

Although we see dogs as our babies, we need to remember they are still animals and when they are afraid or startled, their instinct is to fight to survive. So even the gentlest of dogs can snap and bite when startled but it doesn't by any means label them as an aggressive dog.

'Never wake a sleeping dog' is an important life lesson to teach children, no matter how gentle your dog is. If you allow your children to wake your own dog, they will readily repeat the behaviour and wake other dogs, which might not be as gentle as your own. Your children may have friends round and if they wake your dog, your dog's response could be entirely different to their reaction with your own children. You know as a parent, that children need consistency, therefore establishing rules of behaviour around all dogs, no matter their temperament, will be the best way to keep your children safe and protect your dog's life too.

The saying "Let sleeping dogs lie", was originally coined to mean "Don't stir up trouble". So no matter how gentle your dog is or how sweet and innocent they look while they are sleeping, don't stir up trouble by waking your dog or allowing your children to do so. The startle reflex is your dog's body and brain bracing them for potential danger. So by letting them sleep, you may very well save their life, and that of your child's.

Want to learn how to address this behaviour? See my next article How to Manage Your Dog's Startle Reflex (

Need help with your dog and children. I'm a Kids Around Dogs Approved Trainer and I offer workshops and 1-2-1s to teach them canine body language, consent and how to keep interactions safe and fun. See the link for more info.

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