Gary Gygax answered this question in an EN World interview, and at more length in Slayer's Guide to Dragons.
Originally there were the five chromatic and evil dragons, each with a color that suited their breath weapon, and a sixth good dragon patterned on the Oriental model of that imaginary creature. As it was both or different origination and alignment I decided to empower the gold dragon so as to more closely resemble the potent Oriental sort. So it got more of everything, including two breath weapons.
There came a time thereafter when more metallic dragons were desirable so as to expand the roster of good, Oriental-type ones. Thus all of them were modeled on the gold dragon template, had two breath weapons.
Logically, with metal value being used as the basis for potency, platunum (Bahamut) being the highest, then gold and silver, the sequence should have been platinum-gold-electrum-silver-copper-bronze. However, I thought bronze looked more potent than copper, and skipped then to brass—that metal conveying some not-so-benign connotations.
See http://rpg.crg4.com/originsD.html#dragon for the longer quote.
The 3.5th edition Draconomicon makes no mention about the sleeping/resting behavior of dragons, other than that they do sleep, page 11 states (when talking about a nesting female dragon, emphasis mine):
A dragon egg’s ovoid shape gives it great resistance to pressure, and the female can walk, fight, or sleep atop the nest without fear of breaking her eggs.
Oddly enough, the 4th edition Draconomicon states (page 11):
A dragon can fly for many minutes at full speed, for hours at a reasonable pace, and for days on end if it relies substantially on gliding and updrafts.
However, the 4th edition does not say anything about the effect of flying 'for days on end'. Therefore, I focus mostly on the 3.5th edition book (as the 4th edition book seems catored to offering dungeons/adventures rather than new information regarding dragons). In conclusion, we can not conclude anything directly from both sourcebooks regarding sleeping in the air or on the ground.
However, both books clearly stress that dragons are basically very bad/clumsy flyers (see the amount of effort it takes a red dragon to get in the air on page 18/19, 3.5th Draconomicon).
Therefore, I would personally expect that a clumsy flying creature requires significant conscious thought to stay airborne, meaning that sleeping on the wing would be something that I rule against.
PS: 16 hours of flying? That's a lot of forced march checks ;)
Best Answer
Hope the dragon doesn't come
I'm being a little facetious obviously, but in the fiction of Forgotten Realms for most towns (as opposed to a large city) without nearby adventurers the plan for a dragon defense is to hope they never need to worry about it and evacuate if they see it coming.
And lest you think this is unrealistic and that a town would try to prepare better, take a look at the plans in place for most towns in the path of a major hurricane. In the industrialized west, there are things in place like FEMA or an equivalent and insurance, but those are mostly for cleanup afterwards. There is no plan to try to stop a hurricane and most towns as opposed to large cities don't even have real resources available to try to mitigate or really allow people to reasonably ride it out in place. For most towns that are in hurricane paths, the plan is to hope it doesn't come, and evacuate if one does come.
A large city is likely to have adventurers available, or at least a standing army with a couple of higher level people in the army. But a small town is not likely to have a standing army or high level citizens. If they know the dragon is coming well ahead of time, they will probably try to either hire a band of adventurers or hire mercenaries (and the line between those two is very thin). A town that sees it coming well in advance that is underneath a higher level of government or has larger allies might call for help from them. But if they have little notice, likely all a town without adventurers nearby can do is evacuate and rebuild after. If they have very little notice, the evacuation will probably look like panicked people running all over the place.