Each of the actors eventually left the series for various reasons (career, health, whatever). When William Hartnell (the original Doctor) bowed out, the series was doing extremely well and no one wanted it to end. The storywriters made up the bit about the Doctor being able to regenerate his body in extreme emergencies, and so we ended up with Patrick Troughton as his replacement. Tom Baker (#4) played the part for almost seven years, and was so severely typecast that he couldn't escape the sci-fi/fantasy genre; his most recent performance was as an elf in Dungeons and Dragons. On average, I only get a couple hours of sleep per day, anyway. Poser 5, if and when it gets here, isn't going to affect my sleep cycles at all.