The moon. The moon has a long association with being crazy (lunar being the basis for a lunatic). It shines at night, when you go to bed. The moon’s gravity creates the ocean tides and when the moon is different colors it can mean bad weather is on its way.
A similar problem can be found in L.A. Graham’s Ingenious Mathematical Problems and Methods with a range of 1 to 9, but the principle remains the same – the numbers with the smallest difference produce the largest product. You start out with the highest two digits, 7 and 6, then attach 5 and 4, putting the smaller of the two digits with the larger number, giving you 74 and 65. The next two highest digits are 3 and 2, giving you 742 and 653. Finally, you add the 1 to the lower number. Page 80 has the details of that solution.
What must be in the oven yet can not be baked? Grows in the heat yet shuns the light of day? What sinks in water but rises with air? Looks like skin but is fine as hair?