Ryan has a rare condition known as sexsomnia, causing him to perform sexual acts in his sleep which he then has no memory of.
Whilst still sleeping, Ryan, 28, regularly wakes up Dee, 25, for sex - but has no idea the next day he has done so.
The couple, from Basingstoke, Hampshire, knew they would have trouble conceiving after Mrs Harris was diagnosed with polycystic ovary syndrome. They were due to start fertility treatment when they found out Dee was pregnant.
She said she'd had to keep records of when they had made love for her consultation with a specialist, and when she looked back on her notes, realised she had conceived on a 'sleep sex' night.
"When I looked back, it was clear I'd conceived on a night we had 'sleep sex' as we call it - which means I'm half-awake and Ryan is actually totally asleep. That night Ryan took me by surprise. It put more of the fun back into it - and it clearly worked."
"The first time it happened, I sent him a text the next morning, saying 'Last night was great.'
"He replied saying he didn't know what I was on about. He can never remember - but the sex is more intense."
The couple now have a 16-month-old son, Lincoln.
Ryan told The Sun: "I've been talking in my sleep for years but when Dee told me we were having sex as well I thought she was joking.
"She says she likes sleep sex because I last longer, so I think it helped with the baby."
Had you ever heard of sexsomnia before?