I read the recommended norm is between 6 - 8 hours. But it does vary crazily between individuals, sexes, ages, activity levels, etc. I used to dorm with someone who slept four hours a night and was perfectly fine.
Autistics also commonly need more sleep which is unfortunate.
Here's a rant you didn't ask for - It's a fascinating thing sleep really. Take your timing issue. Circadian rhythms supposedly respond to light cue. Light makes your brain active/wake cycle, darkness less active/sleep cycle. I have a delayed sleep phase syndrome which means mine doesn't. In your case, going to bed before the sun rises (should be easy enough with finishing work around midnight) and blocking out all light from your sleeping space (blinds/curtains/what have you) should trick your brain into not recognizing light cue, so not rebooting the brain into active wake cycle. Earplugs may also help you deceive yourself, if your brain is responding to auditory cues to reinforce circadian cycles - there was one article I read (and as someone with a severe sleep disorder I've read a lot on sleep) that found a connection between "dawn chorus" of animals and birds and circadian cues. Humans and their machines add to this - things are generally quieter at night, trucks/cars/machinery starts up during the day, adding to your chorus cues.
The same train of thought often applies to temperature - cooler at night, warmer during the day.
So, keeping your sleeping space cool, dark and quiet may trigger your brain's passive cycle.
Reinforcing your desired circadian cycle with "sleep hygiene" can help to set it, as can supplementing things like melatonin about an hour before your desired sleep time, to hormonally/chemically convince your body to sleep.
Not to continue the rant, but they tell me that daytime fatigue, even when you have been asleep for eight hours, is due to disordered sleep phases. You need the requisite phases of REM, slow wave and so on for "refreshing" sleep and to avoid being tired on waking. If you're not getting enough or any at all of one of the phases, you can sleep all you want and still feel like garbage (I was lectured on this when I was an alcoholic, for example, because drinking until you pass out screws your sleep phases). To this end one of these fitness trackers that also have the option to record sleep states could be helpful to let you know if disordered sleep stages is your fatigue issue. There has been some studies done in clinical sleep labs to test the accuracy of these activity monitors compared to generally more accurate medical equipment - to mixed results but still interesting.
Also, and I'll stop ranting after this, it's normal to feel tired for a few weeks after you significantly shift your cycle. After that time, if you continue the schedule routinely, your brain should adapt and you'll start to feel less tired and find sleeping easier. 
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Alexithymia - 147 points.
Low-Verbal.