Hello and hope you are doing well.
You could be having an entity called delayed phase syndrome, where in the biological clock is reset and occurs late. It is a circadian rhythm disorder. There is a circadian rhythm for all living organisms. It is a 24 hour cycle rhythm and the biological clock within that individual adapts to this rhythm. This can be reset by bright light therapy, sleeping pills and sometimes by sleep debt. In sleep debt, though you happen to sleep late, you condition yourself to get up early; this in turn will help you to sleep early. So, try this, but if your symptoms still persist, I would advise you to discuss your symptoms with your primary care physician who may then refer you to a sleep specialist for further evaluation.
Hope this helped and do keep us posted.
I used to have a similar problem sleeping at night, I'm afraid of the dark. Particularly thinking I'm alone in the dark when I'm not actually alone. I used a lot of relaxation tips- like meditation and baths before bedtime to help me relax into sleep. I used to be able to manage 6 hours this way from (typically 4am-10am). Since I've been with my husband it's been up to 8.5-9 hours a night- but really it comes back to stress for me. I control my stress, I control my anxiety and fears.