I can't be certain this is accurate, but this was taken from an article online:

NBC’s problems with “The Jay Leno Show” have also affected the ratings of “The Tonight Show With Conan O’Brien.” Mr. O’Brien averaged 2.8 million viewers at 11:35 p.m. from June through December, according to Nielsen, 1.5 million of whom were ages 18 to 49. In the television season that ended the month before Mr. O’Brien’s start, Mr. Leno averaged 5 million viewers at 11:35 p.m., 1.8 million of whom were 18 to 49.