PER3 Polymorphism Predicts Sleep Structure and Waking Performance

The latest edition of current biology holds an article about the connection of VNTRs on genes and sleep behaviour. (Current Biology 17, 1–6, April 3, 2007)
The genetic background about sleep and waking patterns is largely unknown. This paper writes something about it; not everything is understandable for me, as I have a limited background in biology (actually, no background in biology), but with the use of wikipedia I could draw some conclusions.
Individuals were monitored in their sleep-wake cycles, after which some intensive physiological tests were done. This was done in normal conditions, and in conditions of sleep loss. The persons, selected on basis of their genotype and homozygosity for the PER3 -gene, showed no significant differences in bed time, wake time or sleep duration.

Note: PER3[4/4] means that the person is a homozygote, with 4 repeats of the characteristic amino acid.

In their normal patterns, there was no significant difference in the different stages of sleep (REM sleep, stage 1 sleep, stage 2 sleep, total sleep time), but

"PER3[5/5] subjects fell asleep more readily than PER3[4/4] subjects"

When the PER3[5/5] were kept awake for a long time, the subjects performed worse than PER3[4/4] persons on spatial, reaction-time, and logic tests, especially in the late night and early morning hours:

"Most strikingly, PER3[5/5] homozygotes performed very poorly during the hours after the melatonin midpoint. The decrement in waking performance in the PER3[4/4] homozygotes was far less. These major differences in performance between the two genotypes occurred during the late-night and early-morning hours, a time known from both laboratory and field studies as the nadir of the circadian timing system and during which performance is poorest and sleep propensity at its peak. "

"The PER3 5-repeat allele, which is the less frequent one in most ethnic groups, has been associated with extreme morning preference, while the 4-repeat allele has been linked with DSPS in our previous study."

DSPS is a delayed sleep phase syndrome; people with this syndrome tend to fall asleep late at night, and have difficulty waking up in the morning. Furthermore, for a lot of these persons it doesn't matter at what time they go to bed, because they fall asleep at approximately the same time anyway. DSPS is a syndrome from the bigger family of Circadian rhytm sleep disorders; a well known member from this Circadian rhytm sleep syndrome is the jet lag. This may mean that there's a problem with a part of the brain that produces melatonin, which receives information from the eyes about light and dark.

These results, among others,

" (...) led us to consider it as a candidate for mediating some of the marked individual differences in sleep-wake regulation. These individual differences include the preferred timing of sleep-wake cycles, the structure of sleep, EEG patterns during sleep and wakefulness, and their response to sleep loss and circadian-phase misalignment."
"Our results indicate that the PER3 polymorphism may contribute to the marked individual differences in performance decrement during sleep loss."

All in all, this may signify that there's a relation between day- and nightpeople.

"Conclusions
The effects of the PER3 polymorphism on SWS (slow wave sleep), SWA (slow wave activity), and the decrements of waking performance during the biological night, as observed in this study, are significant and substantial. This implies that this polymorphism may be an important marker for individual differences in sleep and susceptibility to sleep loss and circadianphase misalignment, which are major causes of health problems and accidents in our society."

Of course, there are still a lot of open questions: what happens for example with heterozygotes? Or with a smaller number of VNTRs on the PER3 gene? Is there a connection, or is it just a correlation? I'm not in the position to answer these questions, as I could barely understand the article ;).

From: Viola et al., PER3 Polymorphism Predicts Sleep Structure and Waking Performance, Current Biology (2007), doi:10.1016/j.cub.2007.01.073