Matt Smith is suffering some sleepless nights because of the stress of working on the show.
The 30-year-old revealed his struggle in a new BBC documentary, which sees him sitting down for afternoon tea with Victoria Wood.
Matt tells her: “I have had some trouble sleeping, I think because of learning lines and work and stuff, so after six o'clock I have started drinking decaf tea, which my dad was appalled by.
“I asked my dad for a cup of decaf tea and he looked at me like I was an alien.”
In a lighter moment, Matt also said he thought Doctor Who had a “space kettle” he took when time travelling around the galaxy in the TARDIS.
He also told about how tea was important to his daily life.
Matt said: “These things bed themselves into the fabric of our society.
“If I was making a tea advert I would want to communicate about tea is that it can console you, it can start your day, there is the warmth and the ritual and you can share it, you make someone a cup of tea and you offer it to them.
“You have got to boil the kettle, you have got to ask them what they want and engage with them.
“I love the ritual of a cup of tea in the morning, I love tea and a scrambled egg sandwich or tea and a fry up.”
Watch a clip from the show:
Victoria Wood's Nice Cup Of Tea BBC One Wednesday 10th April 2013