Heidi Thomas: 'My dramas are the new Sunday lunch But why is period drama so popular? Of course, there are the frocks. We should pause for a moment, to talk about the frocks. A very senior actress on Cranford once said to me "Why must they call it costume drama? I wear a costume in everything I'm in. Quite. But it would be churlish to suggest that the visual trappings of period shows are anything less than central to their charms. Ralph Wheeler-Holes, the designer for Upstairs Downstairs, has created the most stunning wardrobe for our central players - uniforms and feathered peignoirs, silken gowns in cream, champagne and amethyst. The hair and the nails are immaculate, too. You can smell the Mitsouko perfume, sense the brush of mink - it's aspirational, uplifting, the very stuff of dreams. And I'm as seduced by this as anybody else. But, perhaps ironically, I believe that the real appeal of period drama is that it deals with timeless issues. The stories I love to tell on screen may have a historic setting, but the human experience comes first, with all of its emotional baggage - love, jealousy, betrayal and labyrinthine family relationships. Upstairs Downstairs is set in Belgravia in the late Thirties, but a marriage is crumbling and a young man can't get a date with a gorgeous girl. Call the Midwife is set in the East End in 1957, but babies are being born naked and bawling, and young girls are grappling with their self-esteem - and it was ever thus. The past is not such a foreign country as we are taught to think. Nevertheless, looking back also affords us some perspective. In the past, the moral and emotional stakes were often higher than they are in our liberated times. A simple kiss had huge significance; two hands touching on a staircase was a thrill. Divorce was a scandal, while contraception was effectively unknown before the 1960s, and abortion illegal, so illicit sex carried stupendous risks. The characters in Call the Midwife and Upstairs Downstairs grapple with all of these dilemmas - the emotions they experience are recognisable to a modern audience, but the drama is intensified by the historic setting. People have far less to play with, and much more to lose. This is the lure that draws me back to my computer, time after time, and keeps me up and writing through the night. Call the Midwife was accused by some critics of being "chirpy," and looking at the past through rose-coloured spectacles. We were all a bit perplexed by this as, although the tone was often uplifting, we also featured syphilis, prostitution, maternal mortality and incest. My scripts were based on Jennifer Worth's memoirs, which drew deeply on her actual experience of a very real world, where pain rubbed shoulders with ecstasy, and filth was tempered with hope. Life is like that - messy, sometimes, and impossible to pigeonhole. That is what drama should reflect and celebrate. A little loveliness is not a crime. It can be the reason that makes people go on living. There is no formula for success. If there was, I would bottle it, or write it down, and sell it to the people who keep asking for the secret. In fact, I'd probably sell it to myself, for I have no recipe, no map, no chemical equation. There is a strange thing that happens sometimes, when the hairs stand up in a shudder on my forearm, and I suddenly "know" that a thing will work. But this occurs randomly, and seldom, and all the rest is instinct. Adapting a book, or building on historical sources, is a bit like being a water-diviner, wandering wildly over fields and waiting for your hazel rod to twitch. The spring, the source of magic, is underneath the sod and, once you chance upon it, you roll up your sleeves and dig. I suppose, for me, drama is about a process of response: how I respond to an idea or a book; how actors, directors and designers respond to my scripts and - beyond all else - how the viewers respond to the end result. The attention, laughter and tears of the audience are the final and most important pieces of the jigsaw puzzle. The viewers are not a statistic to me. I don't picture them as faceless millions, but as their individual selves. They are my neighbours, my relations, the girl behind the counter at the paper shop, my son's schoolfriends, the nurse who did my blood test at the clinic. I respect them all, and do my best to serve them. Because "period drama" has become synonymous with something else - Sunday nights. I have been mercilessly ribbed by my nearest and dearest since being dubbed (by the press) "The Queen of Sunday Night," but I believe that the recent popularity of all period drama does have something to do with the way it is scheduled. Sunday, now, is often the only night of the week when families sit down en masse - parents, siblings, teens and younger children. Viewing has become a little ritual, perhaps with wine or a box of unnecessary cakes. People aren't just making an appointment with a programme, but making time to be together. In some ways, in our fractured and spiralling society, period drama has taken over where Sunday lunch left off. It makes a community of us all, drawing us together in our homes, and sending us back into the world with a common talking point. Oops. I'm coming over all rose-tinted and escapist. And I wouldn't have it any other way. Upstairs Downstairs" continues on BBC One on Sunday. Call the Midwife" will return next year.