Did you know that the largest prime number found to date is

2^25964951 - 1

which is over 7.8 million digits long?

Of course, that's a Mersenne prime, and it's harder to find large non-Mersenne primes. The largest non-Mersenne found to date is

27653 * 2^9167433 + 1

which is only a measly 2.7 million digits long.

(OK, OK, I'm just getting my own back for the sarcasm! But if you find this helps you with your insomnia, Neil, I can always keep going ... [

])