“For me, now, a puzzle emerges. What, then, fuels insomnia – fear or anxiety? Anxiety, everyone says. Anxiety, my hypnotherapist says; you are safe in your bed yet your heart is racing as if a tiger is present. You must learn to see that there is no tiger.

But there is a tiger: sleep deprivation. Sleep deprivation isn’t a perceived threat but a real one, like thirst or starvation. It is the fear of not sleeping that raises the heart rate and tenses the muscles; fear, not anxiety. Here is where insomnia becomes intractable, because it deploys fear to act like anxiety. Where fear is a response to an external threat, insomnia is almost unique in giving rise to a fear that then causes the external threat. Being afraid of the saber-tooth tiger is what makes the tiger keep coming back – not seem to come back, but in fact come back. It is no use to say ‘don’t be afraid’. There is a tiger in your bedroom, you ought to be afraid.”