Deprived of sleep, the Swiss yachtsman was at the point of delirium.
Bizarrely, the lack of sleep was part of an exercise taking place in his boat ... but on dry land.
It was preparation for days alone at sea, cut off from the world, as well as nights where sleep is short and fleeting.
On Wednesday (November 1), Koster will leave Gran Canaria and sail 5,000 kilometers to Le Marin, Martinique, on the second leg of the Mini Transat, a race he has attempted twice before.
He finished sixth on the opening leg between La Rochelle on the Bay of Biscay and Gran Canaria.
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A day and a half awake
During his last voyage in the fleet in 2015, he was approached by the psychologist Thomas Theurillat.
'The boat wasn't in the water and there wasn't much time before we left so instead we did it in the yard,' recalls Koster. 'For 36 hours I stayed awake to see how I reacted, to see how I could be pushed with sleep, and see what I was capable of when I was tired.
'There were questions like 'what can I do about it, what are the signs I'm getting too tired?' It was pretty hard and it gets to a point where you're not capable of doing things properly anymore.
'I recall taking one measurement four times to remember what number I was reading. You get pretty stupid when you're tired.
'You make big decisions on the water and you need to have slept to have done that.'
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Theurillat runs One Day -- a coaching method centered on one individual, with one coach addressing subject in one day -- and works with everyone from Olympians to paragliders and businesses.
'Normally, you have the boss and the employees,' he explains. 'The boss normally has to think about doing the right thing, and the employees then do it. Simon has to do both.
'Being a solo sailor is a big challenge: to prepare to be alone or to relax during the night. It's not enough just to prepare the boat and the body, you have to prepare the brain.
'When we're drunk or tired, the brain doesn't have a back-up if you make mistakes. If you make mistakes, you have no way of knowing. So you need to understand at what point those mistakes come in.'
Koster estimates he will average four or five hours sleep across any 24-hour period throughout the race.
For 2017, he has worked more closely with Theurillat on the water, playing out sessions while sleep deprived so the decision-making process becomes automatic when he's at the point of exhaustion during the race.
The rules of the Mini Transat state that sailors are not allowed to contact the outside world.
Unsurprisingly, Koster describes the race as 'much more of a mental battle than a physical one.'
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But in some ways, Theurillat is always there with him. A walk around his boat, throws up all manner of reminders.
As he steps outside the vessel, he is met by a sticker with the words, 'how long do I have to be outside for?' and 'have I got two or three bars to eat in my pocket?'
At the wheel, is another: 'Do I actually have to steer or can I go to sleep?'
They are little reminders that carry significant meaning. For, do you need to steer or will autopilot do a better job, particularly if you're tired.
Theurillat points out these are Koster's words and thoughts and not his own, ensuring the sailor is making as many correct decisions as possible.
'We see peak performance like a cake, there are so many layers: the boat, tactics, the body,' says Theurillat. 'One part of that is taking time to know what you're going to do. If you start doing something without a plan, it's easier to go in the wrong direction.
'It's easy to make the right decision when sitting on the shore over a coffee.
'But not on day 10 on the water when short of food and tired. So since the first leg, we've spoken about the decisions he's made, what he'd do again and what he'd do differently. With Simon, it's about trying to have positive pictures about what lies ahead.'
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Koster also keeps a log book into which he makes entries twice every 24 hours, asking him questions he believes will be pertinent when he revisits the words later in the race. As he puts it, 'it's just another way of taking a step back.'
For him, the psychology has been a massive boost. First and foremost, 'I don't get myself in the same gnarly states as I did before, I'm no longer making big mistakes.
'I might now sleep more than others in this race but my decisions will be better, I'll take the right path and that's much more valuable in terms of time.'
It's not just Theurillat's expertise in sleep that have helped Koster. He's also offered guidance on how to stay mentally strong during days alone on the ocean.
Koster says that area of sailing has never really bothered him. But not being able to consult with someone else around ideas and decisions leads to doubt.
He now has greater self-confidence and more belief in the team that has put together his boat for the voyage.
There are times he has been pushed to the limit but, he says, never to the point of breakdown.
'I have no problem shouting and hitting the boat,' he says laughing. 'The only problem there is you lose your voice but there's no one to speak to anyway!
'I know some are worse, there are some [of an 82-sailor fleet] that have actually put dents in their boom from hitting it in frustration! I think I can keep that in check.'
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