All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First we ask Allaah to reward you for being keen on performing your prayer at its fixed time. You have to seek the help of Allaah and ask Him to enable you to wake up for those two prayers, one of which is 'Asr which is the middle prayer (mentioned in the Quran) according to the most preponderant opinion of the scholars. Indeed Allaah has ordered us to perform the prayers on time especially this one ('Asr). Sleeping after the time of the prayer has started is not permissible if you fear that you will not be able to pray on time. If you sleep before the time of the prayer starts, you have to take the means to wake up like setting the alarm clock on and so forth, otherwise you are sinful. But if you take the measures to wake up but are unable to wake up, then you are not sinful as the Prophet said: "Being overcome by sleep is not considered negligence." [Muslim]
Among the things that would help you wake up is to remember the greatness of Allaah before sleeping and remember the importance of performing the prayer at its fixed time and to be determined to perform it on time. In addition, you should remember the seriousness of neglecting the prayers, Allaah Says (which means): {Then, there has succeeded them a posterity who have given up the prayers and have followed lusts. So they will be thrown in Hell.}[Quran 19:59]. May Allaah enable us all to mention Him, be grateful to Him and worship Him as He should be worshipped.
Allaah Knows best.