Hi Iam,
Many members, to my knowledge have sleep problems when in POIS. I don't have that myself, but I can tell that sleep is not as effective as it is usually in getting rid of my fatigue when it is related to POIS.
Some members says their sleep cycle is broken when in POIS, like they would sleep during the day, and they are awake at night. Others have insomnia.
The exact reason is not known. All I can say is, we all have 24 hours cycles, called circadian rhythms, for almost all our biological functions, neurotransmitters levels, and the like. For example, relating to sleep, we have the melatonine cycle - our own production of melatonine peaks every day during the night ( or brain produce melatoni, in the pineal gland). The production of melatonin bring a good sleep quality. When changing time zone while traveling, it is the dephasing of our melatonin circadian cycle that causes jet lag - this melatonin production peak does not fit anymore with night time, and it takes a few days to our physiology to adapt.
From there, I guess that POIS acts as a kind of "metabolic jet lag", causing a deep dephasing of many our neurotransmitters, including, maybe, melatonin. This would explain the sleep disturbance.
It is good to note that the melatonin pathways are directly connected to the serotonin pathways, in our brain. Many members have symptoms that suppose a dysfunction in the metabolism of serotonin, like anxiety, lack of motivation, and depressive feeling. If POIS can disturb serotonin pathways, it is no surprise that it can disturb melatonin production as well. Like with jet lag, it seems that our body needs a few days to get back in its tracks.