From your descriptions it looks as if the inability to achieve erection has been after an alcoholic binge. Possibly, this has led to performance anxiety and thus the inability the second time. This is known as psychogenic ED (Erectile Dysfunction). Since you are normal otherwise, with sexual hairs and proper height and weight, it is unlikely that you may have congenital syndromes associated with ED like Gonadal dysgenisis etc. ED is defined as a consistant inability to achieve and mantain a erection for a sufficient satisfactory intercourse.
Apart from psychogenic ED, Vasculogenic, Neurogenic, Endocrine causes and drug related causes are the known factors.
Certain tests like noctornal penile tumescence (NPT) testing and doplar scans and cavernosgraphy to identify venous leaks are used in practice.
Psychotherapy, vacuum devices, intracavernous injections and sildenafil (viagra), are the different modes of treatment based on the clinical and laboratory tests. Surgery maybe indicated for venus leakage.
If your having night erections, the possibility of anything abnormal is unlikely. However, it would be prudent to consult a specialist to treat ED.
Dr.M.Jagesh kamath MBBS. Dvd Consultant dermatologist