Let me answer a bit different question - how do I accomodate my PCs being insomniacs on crack?
DO NOT give them downtime. Even if it does not take up real session time. Yes, that might mean more preparation for you.
If you do give them downtime, be prepared that they absolutely will come up with things of their own to do. Even if it will waste real session time.
If you try to limit them coming up with things, be prepared that they will try to route around it. And there is more of them so they will have more ideas than you (as a GM you should know that already). Know also that curfew, ghetto neighbourhood, plague, severe weather, etc. is invitation, not limitation.
If you absolutely must give them real in-game downtime, they better spend it without any possibility of social contact (locked inside a well that is in the middle of a deep forest on an uninhabited island in the middle of the ocean). That at least will somewhat limit their options.
Edit: some teaching instead of preaching.
Let me give some examples on how to let the PCs release some steam while also indulging your need for some temporal verisimilitude.
If a PC wants some specific item crafted/delivered, he must do something in return (kill some goblins, find a missing relative, vandalize competitor's shop, provide escort for a shipment, etc). Now there are things for PCs to do and additional plot hooks abound.
PCs should never have to wait for very-important-NPCs (prince/archmage is out of town). If the NPC is so important, he should have appointed someone to be in charge during his absence. That someone should be competent enough to take care of immediate issues (subarchmage/prince's main advisor). If noone has been appointed, the important NPC probably is not that important and substitutes can be found.
If PCs want to find some information, there should be specific NPCs that they can go to (tribe's shaman, thieve's guild, village's gossip club). That takes out the unspecified loitering part and gives them someone "real" to talk to. If that NPC gives an estimate on how long it will take to find out, he can also give a wink to how much should be paid to speed things up. 10gp after a week, 70gp tomorrow, 700gp by noon. Or he could give advice on who in the town needs things done while they wait. He, after all, is a person that knows such stuff.
If the PCs want to do business at weird hours, there should be seperate shops that cater to such needs (insomniac shop keepers, blacksmith that is of species that does not sleep, magical automatons that work around the clock). Make them mark up their price accordingly. 100gp in business hours. 200gp after midnight. Revers for selling items.
Do not make PCs search for a specific buyers for magic stuff. There are several ways to solve this.
- There are shops that buy all magic items. They were set up to solve this exact problem. They all use universally accepted price mechanism as set up by your system. Shops that stay open during night might use a different price system.
- If PCs are selling magic items to obtain different magic items, let them craft it themselves by disenchant/enchant rituals. They can even do it during weird hours and noone will object.
- Get rid of magic item selling altogether by giving usefull items as part of loot. It might seem cheesy, but do give it a try. It might require some adjustments to encounters. Mage needs +2 staff but adventure says the boss has +2 greatsword? Boss has an arcane lackey with a staff or vice versa.