My hands are numb by the time I get to the river. It’s 7am. The sun has not yet burnt through the mist, and fog hangs low in Port Meadow, Oxford. Dog-walkers and runners appear and disappear like ghosts. I drop my bike by the water’s edge, and begin to undress.

There are rowers on the river: college teams up at the break of dawn, now sitting miserable as they clutch their oars. It’s 2 degrees outside. The rowers are dressed in thick hoodies and woolly hats. They stare at me as I pull down my trousers and shrug off my coat. They stare as if I’m a kelpie – a creature of legend and darkness, appearing like a bad omen as they huddle in their boat. Perhaps in these early hours they don’t think I’m real at all: just a hallucination, brought on by lack of sleep and the stress of Michaelmas term.

I wade into the water.

It has rained recently. Until then, the river felt warmer than the air, its depths retaining the suns of a long summer. I’ve been swimming every day for the past month. On 22 September, I set out with the challenge to swim every day for one year. My aims are to get in touch with nature, and to spend more time doing what I love. It’s not an easy challenge. Already I’ve met great obstacles: working hours that force my swims into darkness; an open-wound that resulted in swimming with a plastic cast-cover over one leg; and most recently, a common cold, making my skin shiver in protest as I take each step into the water.

The water is still technically warmer than the air now, and I remind myself this as my head plunges down into the ice and resurfaces, gasping for breath.

I’m grinning, and if the rowers didn’t think I was mad when I stripped off my Ugg boots and elephant pants that look suspiciously like pyjama bottoms, they certainly do now. But I can’t help it. My whole body has come to life with the cold, and the feeling is like ecstasy.

I often joke with my mum that cold water swimming is a kind of legal high. But since I’ve been swimming every day, I’ve realised that it’s much more than that. Maybe it is a drug and I’m just hooked, but when I go too long without being in the water, I feel a fire building up inside of me. I feel restless and frustrated. The world becomes too hot, off balance, and I sometimes feel sick with claustrophobia and anxiety.

When I swim, I feel free.

Already, I’m beginning to feel changes. I’m more relaxed than I have been in years – not just when I’m at the river, but in every aspect of day-to-day. I worry less about the small things that until now had begun to slowly but relentlessly take over my life. Already, I have confirmed what I already believed to be true. Swimming is natural; elemental. We are meant to be in the water.

There’s a lot in the news right now about whether cold water swimming can cure depression. As a scientist, I’m very sceptical about establishing cause and effect, but I do know that when I’m in the water, I cannot be unhappy. Every day for the past months, I have felt a rare sensation of pure joy.

You can read more about Ellen’s swimming journey on her blog: 365daysofswimming.wordpress.com