The "be well rested" comment was to make them think that the "tomorrow" in the note was supposed to be 12/27/96, the day after the note was found - this is the Ramsey explanation for why the ransom call never came and they weren't expecting one.
If the note were written on the 25th, which would be the logical conclusion, with parents in the home who heard nothing, "tomorrow" would be the 26th - the day the note was found, which would be expected by the leaver of the note on the stairs the parents used in the morning.
They expected the phone call between 8 and 10 which was supposed to explain the delivery. It was just downright odd to go into "it's going to be exhausting. You better get some sleep". How one looks at that phrase depends on whether or not you believe the note was intended to be a ransom note. I do not. I think it's purpose was to taunt, and that it was also a sort of calling card of the perpetrator.