Last month my friend Martin Hartley was really suffering. As part of the Catlin Arctic Survey, he'd been enduring temperatures cold enough to crack the enamel on his teeth, dealing with excruciating frostbite on his toes, and attempting to get some shut-eye in a sleeping bag with all the insulating qualities of a sorbet. As if that wasn't sufficiently unpleasant, after a week of stabbing pain in his backside he discovered a sewing needle sticking out of his salopettes. Accidentally left in his custom clothing by a seamstress, it was finally extracted with a pair of pliers. Ouch.

Martin summed up his mood by writing "I can think of 20 million things I’d rather be doing than this." I know Martin well, and that really didn't sound like him. To say such a thing meant that he was feeling uncharacteristically despondent. That said, I know that there has been a time on every expedition when I have felt exactly the same.

In a blog post on the expedition website, I attempted to paint a picture of what goes on inside one's head during a long expedition. A few days ago, the expedition's consultant psychologist, Mark Lewis, explained in a follow-up post that these mood swings are called 'reversals' in psychology literature. Mark said that "These reversals can occur predictably and slowly, or very quickly depending on whether the stressors are chronic (acting over a period of time), or acute (momentary)."

The good news is that in his most recent update, Martin is sounding a lot more cheery. He's now got croissants and bacon sandwiches on the brain. Which sounds a lot more like the Martin I know.