Hadith: When any of you sleeps, the devil ties three knots at the back of his head, striking each knot: Ahead of you is a long night, so keep sleeping

عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ أَنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ:
«يَعْقِدُ الشَّيْطَانُ عَلَى قَافِيَةِ رَأْسِ أَحَدِكُمْ إِذَا هُوَ نَامَ ثَلاَثَ عُقَدٍ يَضْرِبُ كُلَّ عُقْدَةٍ عَلَيْكَ لَيْلٌ طَوِيلٌ، فَارْقُدْ، فَإِنِ اسْتَيْقَظَ فَذَكَرَ اللَّهَ، انْحَلَّتْ عُقْدَةٌ، فَإِنْ تَوَضَّأَ انْحَلَّتْ عُقْدَةٌ، فَإِنْ صَلَّى انْحَلَّتْ عُقْدَةٌ، فَأَصْبَحَ نَشِيطًا طَيِّبَ النَّفْسِ، وَإِلَّا أَصْبَحَ خَبِيثَ النَّفْسِ كَسْلاَنَ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 1142]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said:
"When any of you sleeps, the devil ties three knots at the back of his head, striking each knot: Ahead of you is a long night, so keep sleeping. When he wakes up and remembers Allah, one knot is undone; when he performs ablution, one knot is undone; and when he prays, one knot is undone, and he gets up in the morning active and in good spirits; otherwise, he gets up in the morning low-spirited and sluggish."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 1142]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informs about the devil and his struggle with the person who wants to get up to offer the voluntary night prayer or the Fajr prayer.
When the believer goes to sleep, the devil ties three knots at his Qāfiyah, i.e., the back of his head.
One knot is undone when the believer wakes up and remembers Allah Almighty without responding to the devil's whisperings.
Another knot is undone when he performs ablution. .
Then the third knot is undone when he rises and prays, and he gets up in the morning active and in a good mood, given his happiness that Allah has guided him to obedience and rejoicing in the reward and forgiveness that Allah has promised him, along with getting rid of the devil's knots and his discouragement. Otherwise, he wakes up in the morning low-spirited, gloomy, and too indolent to offer good and righteous deeds because he is chained with the devil's shackles and far from getting close to the Most Compassionate.