Luke 9:32 But Peter and they that were with him were heavy with sleep: and when they were awake, they saw his glory, and the two men that stood with him.
I make the assumption that the question is about why it is recorded that Peter James and John were sleepy.
If this is not the question, please elucidate and resubmit the question.
Jesus had taken them up into a mountain and gone apart from them to pray.
Luke 9:28 And it came to pass about an eight days after these sayings, he took Peter and John and James, and went up into a mountain to pray.
The record informs us that the three were naturally tired.
The physical exertion of climbing the mountain and having nothing to do but wait while Jesus prayed, brought on what we sometimes experience – a ‘tiredness peak’ - when it is hard to keep our eyes open.
A record of their tiredness is made elsewhere as follows:
Matthew 26:40 And he cometh unto the disciples, and findeth them asleep, and saith unto Peter, What, could ye not watch with me one hour?
Matthew 26:43 And he came and found them asleep again: for their eyes were heavy.
Mark 14:37 And he cometh, and findeth them sleeping, and saith unto Peter, Simon, sleepest thou? couldest not thou watch one hour?
Mark 14:40 And when he returned, he found them asleep again, (for their eyes were heavy,) neither wist they what to answer him.
Mark 14:41 And he cometh the third time, and saith unto them, Sleep on now, and take your rest: it is enough, the hour is come; behold, the Son of man is betrayed into the hands of sinners.
Luke 22:45 And when he rose up from prayer, and was come to his disciples, he found them sleeping for sorrow,
Luke 22:46 And said unto them, Why sleep ye? rise and pray, lest ye enter into temptation.
It shows that the disciples were normal human beings like you and me.
The spiritual lesson should not be lost on us: -
Matthew 26:41 Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak.
Mark 13:33 Take ye heed, watch and pray: for ye know not when the time is. 34 For the Son of man is as a man taking a far journey, who left his house, and gave authority to his servants, and to every man his work, and commanded the porter to watch. 35 Watch ye therefore: for ye know not when the master of the house cometh, at even, or at midnight, or at the cockcrowing, or in the morning: 36 Lest coming suddenly he find you sleeping. 37 And what I say unto you I say unto all, Watch.
I hope you find this helpful.