إِنَّا سَنُلْقِي عَلَيْكَ قَوْلًا ثَقِيلًا ('We are going to send down to you a weighty discourse... 73:5) ' The word thagil means 'heavy' and the phrase 'weighty discourse' refers to the Qur'an, because the Qur'anic teachings of lawful and unlawful are permanently binding and carrying them out is the weightiest task for human nature, except those for whom Allah makes it easier. According to oft-quoted Ahadith, whenever a revelation descended upon the Holy Prophet t, he went into a trance and felt a peculiar sensation, so that even on an extremely cold day drops of sweat fell from his forehead, and he felt his body has become heavier. The Qur'anic revelation being 'a weighty discourse', his paroxysm was due to this sensation. If the Holy Prophet ﷺ received revelation while he was on his riding animal, it would begin to move the bottom of its neck intensely. [ Bukhari and others ].
The verse under comment prescribes tahajjud prayer on man, so that he may become accustomed to the difficulty of waking at night. This is a struggle against excess sleep and comfort of the carnal self. This exercise will make it easier to abide by the injunctions contained in the 'weighty discourse', that is, the Holy Qur'an.