Anxiety certainly can occur at night. If a metabolic cause has been ruled (sleep apnea, heart arrhythmia, hot flushes associated with menopause or other hormonal issues, etc) then anxiety would be the most likely cause.

The next step would be to see where you are in life right now - issues in your life that need to be talked about and worked through.

Whatever I don't deal with in the daylight hours nearly always haunts me at night.