The 30-year-old actress - who plays a porn star in the new film - struggled to stay awake for the raunchy sequence because she had just eaten a huge Thanksgiving turkey dinner.

She said: "I filmed a love scene on Thanksgiving and I had eaten all this turkey and mashed potatoes."

"I was like, 'I seriously have to film a sex scene right now?' I was so tired from all the tryptophan - an amino acid in turkey that makes you sleepy - that I just wanted to sleep!"

Gellar also claims she knew she had made it as an actress when she saw a porn spin-off of her hit US TV show "Buffy the Vampire Slayer."

She added to Britain's Maxim magazine: "I think there was a 'Muffy the Vampire Layer.' "

"We had a cast viewing in one of the trailers. You know you' ve made it when they make a porn version of you!"