6am to midday start for a 400 allows at least some time to build up a time buffer for sleep on a 400.
I think that what makes 400s often feel harder than 600s is a reluctance to get some sleep on a 400.Liz Crease, in her day used to like 400s that started at 10am the most of all events.
Start at 1 after a good sleep and breakfast. Rattle off 300km by midnight, or 2am the latest (she was fast, knew the routes well and wasn't known to be a fan of extra hilly rides, though she did ride some) then at least a 4 hour sleep before bashing out the last 100.
Whether you'll get sleepy or not depends on so many things. It's nothing like as simple as having rules about start times etc.
Fitness will help a lot. Note that our hero mentioned that he found this ride harder than usual. That's extra physical and mental strain.
400s are usually ridden before 600s, so most will be less fit riding the 400 than on a 600.
6am start means that faster riders can finish around midnight and sleep before going home or at least do the Liz Crease method.
Later start times force everyone into night riding. Altering start times is swings and roundabouts.
Porkers 400 is a 2pm start. Riding it as a permanent with a 2am start has crossed my mind
Going by feel doesn't work.
A wearable device that tracks sleep might help.
My Whoop strap said that I had a poor recovery every night leading up to my attempt to do the Cymru and Porkers double.
The morning of the Cymru, I felt good but Whoop gave me a poor recovery so I wasn't sure I would make the double so I decided to pace myself by perceived effort and see what my power meter and average speed turned out at.
I was just about fast enough and comfortable enough but felt more tired than I thought I ought to be so I knew it would be touch and go. So I decided on enjoying Cymru.
It turned out that I was just about fast enough to do the double, even with my longer than usual stops. But I knew I wasn't quite right.
I slept at the last control of Cymru and finished.
I could have made the start of Porkers. But it would have been tough and I knew I was going fast enough and putting out enough power to complete the Porkers in time. But I also thought that I wouldn't be able to stay awake through the night on Porkers.
So I took my time to ride the 200km home over 2 days instead, spending a lot of the first day sleeping.
I felt good and was going well enough. But the HRV reading never seems to lie. It flagged up that I might not manage the double well before I might have worked it out for myself during the ride, which meant I could make several plans ahead and not run myself down to try a very challenging ride when although I felt OK, knew that I was most likely not going to perform well enough to make it.
Another factor is facilities on the event and what you carry. Ironically, I had to carry things so I could sleep rough, which made me slower on the road and have less time for sleep. Though I could have used hotels and traveled faster and lighter, it would have cost me several hundred quid. (Free speed is expensive!)
An overnight sleep at the start and finish would make a difference.
It's a very difficult issue to deal with because there are so many factors and variables.
But yes. I think that cars have something or several things that are sleep inducing.
If you go onto You Tube, there are "Brown Noise" videos.
It's a steady noise that's thought to help you get to sleep (I use them myself sometimes)
It sounds a lot like car tyre noise. I don't know if it's proven to help induce sleep, but I think it helps me and they seem very popular.
Lowering core temperatures will also send you off to sleep. If you're cycling, it will increase core temperature. I would think that sitting in a warm car would lower it, just as a hot bath before bed does.
Muscular fatigue from tired legs will prompt sleep inducing hormones too.