I came across an interesting reference this week to the practice of vigils in The Hundred Letters of the 13th- to 14th-century Indian Sufi, Makhdum al-Mulk Sharafuddin b. Yahya Maneri. In Letter 28, Sharafuddin outlines what should be the sufi’s daily routine of prayer, and having prescribed the necessary prayers before the night’s sleep, he writes:
If he is diligent, he can perform all these prayers; and he may also impose an added religious duty on himself: ‘They forsake their beds, calling upon their Lord in fear and in hope’ (Q32:16) rightly refers to those who are awake and watchful between sunset and the first watch of the night….Also, to prevent sleep from gaining mastery over him, let him rise at the end of the night’s rest, before dawn breaks, and begin his work afresh—that is the time to beg for pardon, and prayer is more profitable at this time than at night! If at the end of the night he engages himself in the prayer throughout the night, he should seek God’s pardon and recite the Quran. 
The quote from Sura 32 caught my attention in particular, and it reminded me that there was actually a very positive reference to Christian ascetics in the Qur’an (5:82-3)—
You are sure to find that the most hostile to the believers are the Jews and those who associate other deities with God; you are sure to find that the closest in affection towards the believers are those who say, ‘We are Christians,’ for there are among them people devoted to learning and ascetics. These people are not given to arrogance, and when they listen to what has been sent down to the Messenger, you will see their eyes overflowing with tears because they recognize the Truth [in it]. 
But following the reference to Sura 32 in The Study Quran, I found something even more interesting. In context, the full statement covers ayat 15 and 16 and reads: ‘Truly they believe in Our signs who when reminded of them fall prostrate and hymn the praise of their Lord and do not wax arrogant, whose sides shun [their] beds, calling upon their Lord out of fear and hope, and who spend from that which We have provided for them.’  Here is some of the comment appended to aya 16:
This verse was reportedly revealed with regard to some Companions of the Prophet who would stand in prayer from after the sunset prayer (maghrib) until the time of the night prayer (‘isha’; W), while others say it refers to those who pray the night vigil (tahajjud; W). Given that they shun [their] beds], the second interpretation is more plausible….Al-Maybudi writes, ‘Sleeplessness and wakefulness at night are the cause of nearness to the Truth and evidence of the perfection of love, for the first degree of love is seeking conformity, and the attribute of the Truth is that neither slumber overtakes Him nor sleep [2:255}.’ In this vein, the Prophet is reported to have said, ‘My eyes sleep, but my heart waketh.’ (My). 
The reference ‘My’ is to the 11th- to 12th-century commentator, Rashīd al-Dīn al-Maybudī. He cites Q2:255, which closely echoes Psalm 120:4 (LXX): ‘Behold, He shall not slumber nor shall He sleep, He that keepeth Israel.’ But no source is provided beyond al-Maybudī for the final quote attributed to Muhammed. It is obviously closely akin to Song of Songs 5:2: ‘I sleep, but my heart waketh.’
Coincidentally, I came across another reference to what appears to be exactly the same saying in one of Raficq Abdulla’s translations of Rumi. In Mathnavi II, 3547, the speaker says:
The Prophet said: ‘My eyes are sleeping but oh
My heart is alight with the Lord of us all.’
Your eyes may be open but your heart is sleeping,
My eyes are closed, but my waking heart stands erect
Dawn-fresh before the threshold with its own wakefulness. 
 Sharafuddin Maneri, The Hundred Letters, trans. Paul Jackson, SJ, The Classics of Western Spirituality (NY: Paulist, 1980), p. 113.
 The Qur’an, trans. M.A.S. Abedel Haleem (Oxford: Oxford U, 2010), pp. 75-6.
 The Study Quran: A New Translation and Commentary, ed. Seyyed Hossein Nasr et al. (NY: HarperOne, 2015), p. 1013.
 Ibid., p. 1013, n. 16.
 Words of Paradise: Selected Poems of Rumi, New Interpretations by Raficq Abdulla (NY: Barnes & Noble, 2006), p. 49.