There are people who possess an innate sense of time and can wake up whenever they want to - in real life, mind you, not fantasy tales! So I assume Merry was one of those people. It's a talent, an instinct, I imagine, so one either has it or not. It can perhaps be honed by practice, but for those who don't have it at all, it's a mystery, akin to magic!
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'Mercy!' cried Gandalf. 'If the giving of information is to be the cure of your inquisitiveness, I shall spend all the rest of my days in answering you. What more do you want to know?' 'The whole history of Middle-earth..
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