In the US flying biz there are no regs about minimum time from awake to at work. As a practical matter the process of getting ready and getting from wherever you’re sleeping to the airplane take long enough anyone has overcome their sleep inertia.
All the following is simplified to the main case, ignoring a lot of fine print.
Like truckers, our regs essentially require 10 hours off containing an “8 hour uninterrupted sleep opportunity” before work. Where we differ is that we must certify, upon pain of fines, license suspension, etc., that we are fully and adequately alert & legally rested before commencing any workday and any single flight within that workday. Out in the real world when stuff gets fouled up it can take a couple more than 10 clock hours to have that 8 hour sleep opportunity. Which can trigger follow-on delays.
For those airplanes and operations where an inflight nap is an expected part of the process, there’s recommendations (not regulation) to avoid being hands-on for 15-20 minutes after awakening. Compliance is usually pretty automatic just by the nature of the handover process from napping to working. I personally have never done that work so can’t share any insider insight. I recall a well-publicized incident 20+ years ago where somebody fell asleep in the seat (or was legally napping under their applicable regs) in cruise and awoke to seeing a bright light right in front of them heading their way! Chaos ensued as they grabbed the controls and aggressively evaded a collision … with Venus. IIRC only a few people were injured, none fatally. That is sleep inertia at work over a span of seconds, plus taking hasty action before you know you understand the whole situation. Don’t do that.
From having flown with a lot of people over a lot of years in this biz, some people are early morning people and can be monster alert and effective from 4am to 4pm, but not the opposite. And vice versa there are people happy to be finishing working at 3am, but don’t ever ask them to start at 8am; they’re useless.
Despite the “useless” comment just above, the nature of this work requires greater circadian flexibility than somebody with a conventional M-F day job. In the last 6 weeks I’ve been on the van from the hotel to the airport at 3am body time to start my day. Having already showered, shaved, pooped, packed, and snacked. I’ve also landed at 3am body time at the end of a 12 hour workday and not gotten to the hotel room until 4am, and not into bed until 430am body time. And I don’t even fly long haul to crazy different time zones; I only deal with at most 5 of them and usually just 3.
Some folks can hack this, some can’t. One of the filters in the early stages of a flying career is your natural aptitude for handling circadian disruption. I’m a (pre-)dawn patrol guy myself, but armed with adequate sleep can do plenty OK most any time of day or night.