“I don’t know when it began. Perhaps it drifted in at night like fog.”

Debi Gliori has led, by any measure, a productive life. Now in her late fifties, she has five children (three boys, two girls), 80-odd picture books, two Kate Greenaway awards and a space on thousands of children’s bookcases to her name. On the spectrum of achievement she’s nearer the high end than most of us.

And yet that’s not how she always feels. There are times when a fog has drifted in and settled on her, cut her off from the world and left her wondering if she’d ever see the sun again.

“I have had in my life several episodes of depression; from clinical to reactive to postnatal,” she tells me as we sit in her studio beside her home on the outskirts of Haddington, East Lothian. “And they all have a different, horrible flavour.”

Today she is giving me a taste. It’s a cold, clear day at the fag end of 2016 and Gliori is looking forward and back. Looking back because of what is ahead of her. A new book. A picture book entitled Night Shift that is specifically about the fog rolling in.

There’s a measure of trepidation about talking about all this, she admits as we sit here. But also a sense that someone has to. On balance it’s good that it’s her. She makes a fine advocate on the page and in person.

Night Shift is a beautiful book – picked out in black, white and smudged grey – about very dark times. It’s an attempt to describe the experience of depression, how it seeps into and stains everything in your life, fugs up your reactions and emotions, chokes the joy out of living. It’s the story of a girl and her depression (anthropomorphised into the shape of a dragon; Gliori has a bit of a thing for dragons) and it is about both the horror of the condition and how you can learn to cope with it (what she calls “night skills”).

Who is it for? “I suppose the short answer,” Gliori says, “is anyone who needs it, anyone who thinks they might be going into a depressive episode, anyone who is in the middle of one, anyone who is watching someone they love go through it and is desperately trying to understand what they are going through, anyone who has come out the other side, really anyone who is interested in understanding what this illness is. Because it is an illness. I would particularly like to shove it up the nostrils of anyone who says ‘just pull yourself together’ or ‘what has she got to be depressed about’. Any of those helpful things that people say.”

People have said them all to her.

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Today Gliori is wearing dungarees and boots. She is a welcoming host in her garden studio surrounded by books and paint and chalk and her drawings. Her partner Michael has brought us coffee and is now back in the house next door. They’ve been together “like for ever”, she says. “We’re coming up for our 20th anniversary.” All the children have left home so Gliori and Michael are getting used to this stage in life.

Night Shift is a project that started simply as Gliori drawing for herself. It was only halfway through that she realised there might be a book in it. “I started drawing it when I realised I was probably teetering around the edge of another episode of reactive depression because various hard things that had happened in our lives had conspired to make everything quite bloody awful for a while.

“I wasn’t feeling my chirpiest, shall we say? And I thought: ‘Let’s just try to turn the lead of this horrible thing we’re going through into some kind of gold that we can use in future times,’ if only as a diary to look back on and say: ‘That’s what it felt like, but, my goodness, it feels so much better now.’”

So much better now might not quite be where she is at the moment. There is the shadow of fog at the edge of her vision. It’s been three years now since her last episode but depression has dogged her since her twenties when she left art college.

She’s doing her best to keep it away, employing her own night skills. One of the main ones is running. Only every other day, because, she says, “I’m human.”

She has a runner’s physique. “I found pretty early on that intense physical exercise was my way of coping. I’m not someone who gets on particularly well with antidepressants. And I’m a really crap runner. Oh God, am I crap. But I get through it.”

She works, too, sketching and drawing and painting in her studio. “My most successful picture book of all time was done when I was just a mess. I was such a mess. I was in the middle of a horrible divorce, my children were really upset by it, especially my eldest daughter. I had postnatal depression and it sucks the joy out. Nine months of grey fog descending when you’ve got this baby in your arms.

“So out of that came No Matter What, which has sold, I think, one and a half million copies worldwide and is suffused with light and love. And you sort of think: ‘How did that happen?’ I have no idea.”

Life is about coping, isn’t it? For all of us. In her time Gliori has coped with her parents’ failed marriage, her own failed relationships, divorce and parenthood. Coping with depression is another part of her particular package.

The Herald: Taken from Debi Gliori's Night ShiftTaken from Debi Gliori's Night Shift

The first time was the worst time, she says. “That very first depressive episode I had when I left art college back in 1984, I had no idea what was happening to me. I thought

I was going completely and absolutely mad. And my family did as well. At that point I couldn’t work, I couldn’t do anything. I turned into a blob, just this grey shambling thing that could barely get a sentence out.

I closed up my little studio which was a tiny little back room in my house and I didn’t go back in for a year. And that was horrible, especially since that was my income and suddenly that had gone.”

Presumably some sense of your own worth went too. “Yeah, yeah. And my bliss. I love what I do and to lose that … It is so much tied up with your sense of self.

“The next time things started sliding back down into the pit I recognised it. I thought: ‘I know this.’ And I had a strategy for dealing with it. It didn’t make it go away but it kind of beat it back into the shadows ever so slightly.”

The worst thing is, though, it affects others too. “The children do notice, especially as they’ve grown up. My eldest daughter at one point went for counselling because she is a child of the divorce and when she came back I said: ‘How was it?’ Without wanting to pry.

“And she said: ‘Well, one of the first questions the counsellor asked me was: “Tell me about your home situation.” I said: “My mum and dad are divorced, my mum’s got depression and it really, really hurts.”’

“And it brought it home to me. I just thought: ‘Oh my goodness, she sees me as a depressed person. What a horrific legacy to give to a child.’ That was hard to deal with. Growing up in a depressive atmosphere is not a lot of fun.”

We all carry our baggage through life, of course. Gliori might feel she has more than some. She was born in Glasgow into an unhappy marriage that wouldn’t last much beyond her own childhood.

“It was a very unfashionable thing to do when my mum left my dad. Nobody did that kind of thing at all,” she says.

Gliori was six at the time. “The household had not been happy up until that point. There were lots of rows. When I was six my mum and I went down to England and then came back a year later and the marriage limped on for another year and a half.”

When her mother finally split from her father it also started a slow unravel of the relationship with her daughter. “She was really struggling and she couldn’t go anywhere for help apart from the Catholic Church,” Gliori says of her mother. “But the Catholic Church thought what she had done was a mortal sin, so she was caught between a rock and a hard place. It was impossible for her. It was also impossible for me.

“It just got worse and worse and worse and I finally ran away from home with someone who was 15 years older than me.”

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Gliori was just 15 at the time. She returned home after a few days at which point her mother got the authorities in. “The police did the full body search and swabs and all that sort of stuff. I felt so betrayed by my mum that I thought: ‘Right, enough of this. I’m out of here.’

“I grabbed a rucksack and left, moved in with this guy who was 15 years my elder. And that was a disaster.”

Well, yes, Debi, I say. If you were my daughter …

“If any of my children had done this I would have completely lost it,” she agrees. “Fortunately, they haven’t.

“It went from bad to worse. The boyfriend – man friend – eventually went to prison for some minor misdemeanour to do with driving. I was on my own in a flat in Glasgow and I didn’t know how to look after myself at all and, yeah, it was a complete disaster.”

Her flat turned into party central until she woke up one morning with the police knocking down the door looking for drugs. “They turned the place upside down. I’m an inadequate housekeeper. I certainly was then. And they found two roaches [the remains of a joint] from God knows when floating in a beer can. Classy.”

Gliori was eventually given a three-month sentence in Greenock women’s prison. When she emerged she ran away again, from Glasgow to Fife, hoping her mum wouldn’t know where she was.

“My mum hired private detectives to find me and by the time they did I was six months pregnant.”

Gliori was 17 at this point. “My son was born just before I was 18.”

How, I wonder, does she look back on the girl she was then? “With great compassion actually. But what an idiot. How naive was I? How unbelievably un-streetwise to have thought this complete creep 15 years older than me was the best thing since sliced bread. What was I looking for? Was I possibly looking for a dad? Mmm, maybe.

I was patently a complete idiot.”

She stops and smiles. “This is terrifying. Today I’m just about to take a birthday cake around to my 40-year-old son. How can I have a 40-year-old?”

Amid all this teenage emotional carnage there was an artist somewhere inside looking to get out. She’d draw cartoons of the people floating through her life. After her son was born – after a long and difficult labour – she put a lid on her recent past, managed to pull herself together and even get through art college. She’d take her son to lectures.

By third year she realised she wanted to create picture books. And that’s what she would do. Successfully too. She has been remarkably prolific.

But not long after she graduated, that first foggy wave of depression rolled in. It would cover her for the best part of a year. It must have been horrifying having finally gained control of your life at the age of 25 to have it taken away again.

How bad did it get? “I certainly contemplated suicide. It was that bad and that frightening. But I had a child and I just couldn’t do it.”

And if your son hadn’t been there? “Well, the London train used to go across a level crossing a 10-minute walk from my house. I could hear it. That was a dark, dark place. May I never and nobody ever …” She trails off.

The thing about depression, she says, is that it lies to you. “It says that this is endless. You are worthless. You may as well end yourself basically, because it’s always going to feel this bad.”

But in the end the fog rolls back. Life reasserts itself in all its messy, mundane ordinariness. Gliori can point to a family, a life, book after book (one of which made it into the top 10 of the New York Times’ bestsellers list). “Depression is a recurrent theme in my story,” she will later tell me by email. “But it’s nothing like the full story. In fact, I keep trying to force it into the footnotes.”

And so when she isn’t working she’ll be running or walking on the beaches of East Lothian or tramping up hills after Michael. Living her life, in other words.

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There’s a bravery in choosing to be so open about her illness. I wonder, does she feel better having completed Night Shift? “Not personally. It brings quite a freight of responsibility. It’s a very emotive subject but it needs to be discussed.

“It’s about time we got rid of the stigma. There should be no stigma attached to it and no blame. It is quite simply an illness.”

Time for one more story. At the end of the book the girl finds a black-and-white striped feather. When I ask about the feather Gliori starts to tell me about the night Michael had a “heart event” in the middle of the night, his heartbeat going from 50 beats a minute up to 170 and back. The doctors basically had to stop his heart and restart it. When she eventually got to see him in the hospital he had huge burns down his chest from the defibrillator.

“We got back to the house going: ‘Everything feels a bit different.’ And he was understandably blown away. I said: ‘Do you want to go for a walk?’ because it was like the walls were closing in and he said: ‘I don’t know if I can and I don’t know if I dare.’”

They drove down to the beach anyway and went for a short, terrified walk. On the ground Gliori saw a striped feather, bent down, picked it up, not thinking.

Time passed. “Slowly and bit by bit Michael got better,” Gliori says. “Now he’s pounding up Liathach ahead of me and all is well.

“I kept the feather because the feather was emblematic of that day. That day was the black stripe on the feather for sure, but there are other days that are white. You need one to see the other.

“I wouldn’t wish depression on a single soul but in a weird way it enables me to see just the vanilla days of our lives, the ordinary, the days when nothing much happens and you go: ‘Gee. I’m bored.’ Those are amazing.”

She stops, smiles and adds, “That’s my best shot.”

It is more than good enough. Outside the sky is still clear. May it long remain so.

Nigh Shift by Debi Gliori is published by Hot Key Books, priced £9.99.