What is al-Jathum (Sleep Paralysis)?
Question

We often hear about al-Jathum (sleep paralysis), which is a jinni that sits (yajthum) on a person’s chest if he has been neglecting prayer or other duties. Is there anything in the Sunnah of the Prophet (peace and blessings of Allah be upon him) that mentions that? Or is it a myth and fable?
Praise be to Allah.What is Al-Jathum?
Al-Jathum is the kabus (incubus, evil spirit, nightmare) that descends upon a person in his sleep. 
Ibn al-Manzur said: 
“Al-Jatham or al-Jathum is the kabus that descends upon a person at night… the one that descends upon a person when he is sleeping is called al-jathum.” (Lisan al-‘Arab, 12/83) 
He also said: 
“The kabus is the one that descends upon the sleeper at night. And it was said that it is the precursor to epilepsy. One of the linguists said: I do not think that it is an Arabic word; rather it is al-naydalan; it is also known as al-baruk or al-jathum.” (Lisan al-‘Arab, 6/190) 
Reasons of Al-Jathum
The “jathum” may be caused by a physical reason, such as eating too much or taking too much medicine, or it may be due to being overpowered by the jinn. The former may be treated by cupping and reducing the intake of food etc. The latter may be treated by means of the Quran and reciting dhikrs that are prescribed in Islam. 
Ibn Sina said in his medical book al-Qanun: 
Chapter on al-kabus: 
“It is also called al-khaniq (the strangler), and in Arabic it is also called al-jathum and al-naydalan. 
Al-kabus is a disease that a person feels when he is falling asleep and imagines something heavy pressing upon him, squeezing him and constricting his breathing, hence he cannot speak or move, and he is almost suffocated because of the obstruction of his airway. When it goes away, he wakes up immediately. This is the precursor to one of three things: epilepsy, apoplexy or mania. That applies if it is due to physical causes and there is no other non-physical cause.” 
Modern doctors say the same. Dr. Hassan Shamsi Basha has divided kabus into two categories: temporary kabus and recurrent kabus; he regards the former as being due to physical causes and the latter as being due to the effect of the jinn. 
He says in his book al-Nawm wa’l-Araq wa’l-Ahlam (sleep, insomnia and dreams): 
1) Temporary kabus:
This happens for two reasons:
A- Vapours that rise to the brain through the breathing channels when first falling asleep, so the person afflicted feels that he cannot move or speak, or he feels a kind of panic. This is the precursor of a physical epileptic seizure, and it also happens when one is exposed to psychological pressures.
B- Administration of medicines that can cause kabus such as:
(i) Arazrabine
(ii) Beta blockers
(iii) Lifod B
(iv) Antidepressants
(v) After stopping the use of tranquilisers such as valium.
2) Recurrent kabus. This kind of kabus indicates that a person has been harmed by evil spirits. End quote.
Conclusion  
Jathum refers to kabus, which is not a myth or fable, rather it is something real that does happen; it may be caused by physical factors or it may be caused by the jinn. 
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And Allah knows best.