Assalaamu alaikum,
Oops...I forgot to search sleeping on your back.
Hope the following helps:
Guidelines of the Prophet (peace and blessings of Allaah be upon him) concerning sleep
Question:I want to know how the Prophet (peace and blessings of Allaah be upon him) used to sleep. Did he sleep on a bed or on the floor? Did he used to recite a specific du’aa’ when he wanted to sleep?
Answer:Praise be to Allaah.
The Prophet (peace and blessings of Allaah be upon him) used to sleep sometimes on a mattress, sometimes on a leather mat, sometimes on a mat made of palm leaves, sometimes on the floor, sometimes on a bed, sometimes on the sand, and sometimes on a black cloak.
‘Abbaad ibn Tameem said, narrating from his paternal uncles:
I saw the Messenger of Allaah (peace and blessings of Allaah be upon him) lying on his back in the mosque, putting one leg on top of the other. (Narrated by al-Bukhaari, 475; Muslim, 2100).
His mattress was made of leather stuffed with palm fibres, and he had a coarse woollen cloth that he would fold over twice and sleep on.
The point is that he slept on a mattress and covered himself with a blanket. He said to his wives: “Jibreel never came to me whilst I was under the blanket of any one of you apart from ‘Aa’ishah.” (Narrated by al-Bukhaari, 3775).
His pillow was also leather stuffed with palm fibres.
When he went to bed to sleep, he would say: “Allaahumma bismika ahyaa wa amoot (O Allaah, in Your name I live and die).” (Narrated by al-Bukhaari, 7394).
He used to put his hands together and blow into them, then recite Qul Huwa Allaahu ahad, Qul a’oodhu bi Rabb il-Falaq and Qul a’oodhu bi Rabb il-Naas [i.e., the last three soorahs of the Qur’aan), then he would wipe his hands over as much of his body as he could, starting with his head and face, and the front part of his body. He would do that three times.
He used to sleep on his right side, putting his right hand under his right cheek, then he would say: “Allaahumma qini ‘adhaabaka yawma tab’ath ‘ibaadaka (O Allaah, protect me from Your punishment on the Day You resurrect Your slaves).”
And when he went to bed he used to say: “Al-hamdu Lillaah alladhi at’amanaa wa saqaanaa wa kafaanaa wa aawanaa fakam mimman laa kaafi lahu wa laa mu’wi (All praise is for Allaah, Who fed us and gave us to drink, and Who is sufficient for us and has sheltered us, for how many have none to suffice them or shelter them).” This was narrated by Muslim, who also narrated that he used to say when he went to bed:
“Allaahumma Rabb al-samawaati wa’l-ard wa Rabb al-‘arsh il-‘azeem, Rabbaanaa wa Rabba kulli shay’in, Faaliq al-habb wa’l-nawa wa munzil al-Tawraati wa’l-Injeeli wa’l-Furqaan, a’oodhu bika min sharri kulli shay’in anta aakhidhun bi naasiyatihi. Allaahumma anta al-awwal fa laysa qablaka shay’un, wa anta al-aakhir fa laysa ba’daka shay’un, wa anta al-zaahir fa laysa fawqaka shay’un wa anta al-baatin fa laysa doonaka shay’un. Iqdi ‘annaa al-dayna wa aghninaa min al-faqri (O Allaah, Lord of the seven heavens and the exalted Throne, our Lord and Lord of all things, splitter of the seed and the date-stone, Revealer of the Tawraat and the Injeel and the Furqaan [Qur’aan], I seek refuge in You from the evil of all things You shall seize by the forelock [have total mastery over]. O Allaah, You are the First so there is nothing before You, and You are the Last so there is nothing after You. You are al-Zaahir [the greatest and highest] so there is nothing above You, and You are al-Baatin [aware of the subtlest secrets] so there is nothing closer than You. Settle our debt for us and spare us from poverty).” (Narrated by Muslim)
When he woke up from sleep he would say: “Al-hamdu Lillaah alladhi ahyaana ba’d ma amaatana wa ilayhi al-nushoor (Praise be to Allaah Who has brought us back to life after causing us to die, and unto Him is the resurrection).” (Narrated by al-Bukhaari, 6312). Then he would clean his teeth using the miswaak, and recite the last ten aayahs from Soorat Aal ‘Imraan, from the verse (interpretation of the meaning):
“Verily, in the creation of the heavens and the earth…”
[Aal ‘Imraan 3:190-200]
And he would say: “Allaahumma laka al-hamd anta noor al-samawaati wa’l-ard wa man fihinna, wa laka al-hamd anta qayyim ul-samaawaati wa’l-ard wa man fihinna, wa laka al-hamd anta al-haqq wa wa’duka al-haqq wa liqaa’uka haqq wa’l-jannatu haqq wa’l-naaru haqq wa’l-nabiyoona haqq wa Muhammadun haqq wa’l-saa’atu haqq. Allaahumma laka aslamtu wa bika aamantu wa ‘alayka tawakkaltu wa ilayka anabtu wa bika khaasamtu wa ilayka haakamtu faghfir li ma qaddamutu wa ma akhartu wa ma asrartu wa ma a’lantu anta ilaahi laa ilaaha illa anta (O Allaah, to You be praise, You are the Light of the heavens and the earth and everyone in them. To You be praise, You are the Sustainer of the heavens and the earth and everyone in them. To You be praise, You are the Truth (al-Haqq), Your promise is true, the meeting with You is true, Paradise is true, Hell is true, the Prophets are true, Muhammad is true and the Hour is true. O Allaah, to You I submit myself, in You I believe, in You I put my trust, to You I repent, by Your help I strive (against Your enemies) and to You I refer for judgement, so forgive me my past and future sins, what I do in secret and what I do openly. You are my God and there is no god but You).” (Narrated by al-Bukhaari, 1120).
He used to sleep during the first part of the night and get up to pray during the last part. Sometimes he would stay up late to deal with the interests of the Muslims. His eyes slept but his heart did not. When he slept, they would not wake him up; he would be the one to wake up.
When he stopped to rest at night during a journey, he would lie down on his right side, and if he stopped to rest just before dawn he would prop his head up on his forearm, resting it in his palm. This was narrated by al-Tirmidhi.
His sleep was of the best length and it was the most beneficial of sleep. The doctors say that it is one-third of the night and day, namely eight hours.
See Zaad al-Ma’aad, 1/155
0
And I wanted to ask what I should do if I made a mistake during my prayer...like saying the sjahada at the 3th raka'a when actually it should be at the 4th...and other things like that. Sorry if these questions allready have been asked! 
