All praise be to Allah, peace and blessings be upon Prophet Mohammad.
Offering prayers on their fixed times is among the greatest acts of worship by which one gets closer to Allah. Ibn Mas`oud (May Allah be pleased with him said: ” I asked the Prophet 'Which deed is loved most by Allah?" He replied: "To offer prayers at their early (very first) stated times.' " 'Abdullah asked: "What is the next (in goodness)?" The Prophet said: "To be good and dutiful to one's parents," 'Abdullah asked, "What is the next (in goodness)?" The Prophet said: To participate in Jihad for Allah's Cause." 'Abdullah added: "The Prophet narrated to me these three things, and if I had asked more, he would have told me more.” {Bukhari&Muslim}.
Therefore, a Muslim should take precautions in order to offer prayers on their fixed times such as setting the alarm clock on time, or asking someone else to wake him/her up on time.
However, if he slept after the time of a certain prayer had started , and was certain that he wouldn`t be able to offer it before its time ends, then he is considered sinful, and should repent to Allah.
Nonetheless, if he had slept before the fixed time of a certain prayer started , then he isn`t considered sinful, but he misses a great reward. However, his fasting is valid in both of the aforesaid situations if Allah wills.
[Al-shirwani Ala Tohfat Al-Minhajj] (1/407) states:
“If he had slept before the fixed time of the prayer started (and its time ended), then he isn`t considered sinful. Also, he isn`t considered sinful if he had known that the fixed time of prayer will end while being asleep.
However, he is to make up for it at his convenience. If he slept after the time of prayer had started, and was sure that he would wake up before the time ends, then he isn`t considered sinful even if the time of prayer had ended before he woke up.
However, it is disliked for him to sleep after the time of prayer had started unless he was so exhausted that he couldn`t resist falling asleep Moreover, if he was sure that he wouldn`t wake up within the fixed time of prayer, then he is considered sinful.
As for the person who abandons prayer out of laziness, his fasting is considered valid by the will of Allah, but he had committed one of the major grave sins which entails sincere repentance to Allah and making up for the prayers that he had missed.