re: Is it more energy efficient to use a portable A/C in the bedroom at night over central air

I have one in my bedroom. Our bedroom windows face west so it gets pretty warm in there every evening. I usually turn it on around 6 every evening and off when I get up in the morning.

I have a 6500 btu window unit that I use for camping. I have used in 4 or 5 times when my main A/C was down for some reason or another (always Friday evening) It will do a good job in a pinch for your 400 square foot area until you get it back up and running. Even if I did not have it for another purpose I have gotten my moneys worth to have it as a stop gap measure.

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like to sleep with the temp around 68 at night

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I did too when I was a BFF. 75 and a fan is fine now.

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You spend less energy keeping your house at temp than you do at trying to catchup to a temperature.

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common misbelief. also, bringing temps down creates long run times thats great for humidity removal. considering most people have too large a system that can be poor for humidity removal, long run times can be beneficial.

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single hose portable units are trash.

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You are getting DVed but you are exactly correct though I wouldn't call them trash just very inefficient. This is why they have a DOE BTU number much lower than the actual BTU the unit runs.

A single-hose unit (as you said) is pulling all its air from indoors so all the air that runs over the condenser coils comes from inside and is blown out the window. That has to be madeup from somewhere. Attic doors, drafty windows and doors, bathroom vents, or wherever else the least resistance is. This also impacts humidity because they are inefficient at dealing with humidity too, they do blow some humidity out the exhaust but they are steady pulling it in.

Even if you own a single hose there is hope you can simply DIY it into a dual hose.

Get some aluminum HVAC tape and some pink rigid insulation (duct board if you have a source that is free, it is a lot more expensive than rigid insulation) and make a plenum that encloses the air intakes sealing it with the tape, you cab use hit glue to build a more durable plenum if you have it available. Then cut a hole in the plenum and connect a vent hose and vent it out the window. There are a ton of options but one that makes it easy is to get a portable AC hose kit from Amazon and use that. Get one with the end and grate in the kit. The use some more of your pink insulation and replace the plastic window adaptor from your AC unit. Cut holes and glue the input and output on opposite sides. You can use some spray paint to paint the insulation white outside and/or inside to make it less visible. When you are done wrap the exhaust hose with bubble wrap so it isn't constantly radiating heat back into the room.

Before I broke down and bought a mini-split for my work shop I had two portable A/Cs and even though they could keep it in the low 70s it was never comfortable due to the humidity. I "converted" them to dual hose and I could keep it 4-5 degrees warmer and it felt so much cooler so it wasn't fricking up my lumber. I was able to check my power usage on the app on days I didn't work in the shop so no other power was being used (except a few watts here and there) and my power usage went down over 30% and it was more comfortable to boot.

If you are buying one now buy an inverter based A/C with dual hoses, just spend the extra money I promise you won't regret it. If you already have a single hose or are on a tight budget convert it to a dual hose and insulate the exhaust hose, it will increase the efficiency and your comfort significantly. If you just want something to keep you from melting in a power outage and you only have a portable gen/set a single hose will get you by and is easier to roll in and out of a closet than dealing with installing and uninstalling a window unit but don't expect the comfort you are used to with central air even if you have plenty of BTUs looking at the DOE number to meet your requirements.