Vesper Flights. Helen MacDonald

of violence. A slug that ratchets through and slots into your spine before the slow shot begins with an umbrella-towering nimbus of empty pressure that makes you as dizzy as if there really were a storm-cloud of rising air growing, billowing up and outwards until its edges feather and coincide with your skull. Then come two thumbs pressing on your sinus and moving over your jaw, and strange strips of fast pain like summer lightning when you lift a cup, pick up a pen, burying themselves in your shoulder, deep into places which don’t exist until they hurt. And when the pain comes it is one-sided, sometimes on the left of your skull and sometimes on the right, although it is so intense it can’t be kept in either place, and it ripples like a flag cracking in strong wind, or thrums deep like a heartbeat, and sometimes one of your eyes waters, the one on the same side as the pain, and there’s what doctors call a post-nasal drip, which makes the world taste of scalding metal and brine. A few times, in the midst of my own migraines, I’ve had a strong and sudden intuition I’m made out of cobalt: partly it’s that taste in my mouth, partly how heavy I feel, but mostly because the interference in my brain runs sometimes along the lines of those delicate scrawls of blue-flowered decorations on ancient Chinese porcelain. Shipwrecks, bones, pearls. So yes, migraines put me in mind of metaphors, and then more metaphors, and more, for they are always too much in a way that makes them unbearable, all filters gone.

      Thirty per cent of migraineurs experience visual disturbances with their headaches. I’ve had them only once, on a stormy night at a literary festival. I was busily signing books when a spray of sparks, an array of livid and prickling phosphenes like shorting fairy lights, spread downwards from the upper right-hand corner of my vision until I could barely see through them. In textbooks the phenomenon is called scintillating scotoma. It scintillated. I freaked out. I kept signing, kept smiling, gripped the inside of my shoes tight with all ten toes, and worried that I was going to die until the pain came.

      Although they hurt, make bright light a brutal intruder and force me to take to bed and swallow as many painkillers as I am allowed to without harming myself, my migraines seem useful. Their utility isn’t in the pain. The pain is terrible. I hate it. I hate the time it takes from my life, my helplessness in the face of it, the tears soaking the pillows I’m curled around. But migraines remind me we’re not built with the solidity so many of us blithely assume. That the World Health Organization’s 1948 definition of health – a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity – refers to precisely no one, is a sweetly turned phrase more ableist than utopian. That perfection cannot be intrinsic to us, built as we are of chemicals and networks and causal molecular pathways and shifting storms of electricity; none of us are ever in perfect health.

      Migraines are an incredibly common – more than a billion people suffer from them – but highly mysterious neurological condition. We’re not exactly sure what they are, though it’s likely they’re a tendency for the brain to lose control of its inputs, a sensory processing disorder that is partly inherited. We know that meningeal blood vessels around the brain dilate during the headache, and that migraines are associated with activity in the trigeminal ganglion, the base of the nerve network that governs the face and the muscles used in chewing. We know that migraines with auras involve waves of electrical activity across the brain called spreading cortical depression. In the midst of a migraine, not knowing is very much to the point. Pain wipes you free of knowledge, makes understanding utterly redundant. There’s nothing to know or understand. Subjects, objects, fail. All you are is all that is and all of it hurts.

      Some people get migraines most often around the time of their period (three times as many women as men are migraineurs; sex hormones appear to play a role) and the correlation is pertinent to me not only because I am one of those women, but because menstruation is migraine’s closest cousin in my life. There’s no mistaking their occurrence – I bleed, or I curl up in pain and weep – and both involve a suite of premonitory symptoms.

      It took me nearly thirty years to understand the robustness of my premenstrual pattern, but these days I know the week before my period will always include a single day in which I fantasise about murdering strangers, most specifically slow drivers, and another in which everything can reduce me to sentimental tears: supermarket adverts, the polished corner of an oak table glowing in the sun, a pigeon taking off from a hawthorn branch into the wind. For much of that week the voice of my interior critic is as seductive and honeyed as warm baklava. It tells me I am a terrible person and the worst writer in the world, and I believe it. But after decades of bafflement, these states are now something like old friends, and I greet them with a deal of archness.

      The premonitory symptoms of my migraines are exceedingly specific. Two or three days before my head begins to hurt, my fridge fills with bottles of banana milk. I yawn a lot, become unaccountably thirsty. My joints ache. I shop for dark chocolate and sweet pickled beetroot. There’s dust-and-ashes tiredness, and a bad mood so remorseless that even the sweetest birdsong irritates. I can enumerate them now, yet when the headache arrives, it is always a surprise. I never see it coming. These symptoms are aspects of the migraine’s earliest stages, occurring in its prodrome, that part of it which precedes the pain. It turns out that some of the most infamous migraine triggers are not triggers at all – a desire to eat chocolate is just as much part of a migraine as the full-blown headache that follows.

      After the pain subsides, the migraine’s postdrome begins, and it is a peculiar muse of mine. Though it makes me feel weak, muffled, slow and stupid, it’s inside it that writing comes easiest. Whatever is going on inside my brain makes the words flow, the world sharper; it tips me into days that seem newly forged and prone to surprising beauties. I’m writing at my kitchen table right now in the midst of a postdrome, a heat pack draped across my neck and shoulders to unknot muscles locked after two days of hurt. Just after sunrise this morning, I looked out over the back fence of my garden across a field of oats into the valleys and hills around my house, the sky nacreous and the lower-lying ground obscured by luminous mist. For a migraineur in fear of bright sunlight, the slow slide into autumn’s softer days and earlier nights is an enormous relief.