Hi, Sleepiness or tiredness after sex is normal but you are feeling it too much and that also even on the next day and your another problem is a weak erection. However, if you would have visited my clinic I would rule out any physical causes and would get your testosterone, thyroid and prolactin hormone levels checked. If that comes out to be normal then there may be psychological reasons also. You can take a good protein supplement and a multivitamin. Ayurvedic medicines like Tentex Forte can help but that had to be taken after consulting your doctor. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Shantnu Sharma, Ayurveda Specialist