I'd 2nd a hard, interesting (but useless in nearly future) language. For me it's Japanese now. I couldn't learn much more hieroglyphs, knowing I won't use them frequently.
The third is a multilingual text when I try not to read footnotes (for French, German and other language sentences and phrases).
Then the worst are books in native language (Russian). It doesn't help, even worse, I can be reading all night. Much better is the reading an English text with many of Latin words. It's almost multilingual. What about You?
Nearly an X-post in Russian http://sukina-docha.livejournal.com/6522.html
UpD (Today is Sunday,26ofJune,17:00GMT) Thanks to ALL
may be I did not proper expressed my thought. Nobody forces oneself to sleep by mind-boring reading . Eh?
