A simple explanation can be found when you consider sleep to be a state when the body suspends sensory and motor activity. The voluntary muscles are quiet or inactive. Naturally the involuntary muscles continue to function but your breathing and heart rate change. But the most visible symptoms of PD are often in the voluntary muscles.

One theory is that this is a period of discharging so that the mind and body can begin the next wake cycle.

This appears to happen in Parkinson’s to some extent as the tremors seem to stop once the person has actually fallen into a deep sleep.

When Michael J Fox is playing hockey – a fast game when the body has to move and respond from training which produces instinctive responses – the activity is rather like Forced Exercise which has been found to reduce PD symptoms and a reduction in dosage of PD medications for up to 2 weeks if done properly. In this example, the brain is actually trying to find a healthier balance (that’s the short version)
http://parkinsonsfocustoday.blogspot.com/2009/05/forced-exercise-to-relieve-parkinsons.html

You can find a good explanation about sleep at everyones’ resource, Wikipedia:
http://en.wikipedia.org/wiki/Sleep

You might also like to read various topics about sleep at the National Sleep Council
http://www.sleepfoundation.org/let-sleep-work-you

About Pain and sleep:
When you go into the 3rd and then the 4th stages of sleep – the delta stages – the body has a chance to do some repair to relieve the pain. Some people with chronic or acute pain are not able to reach this stage of sleep.
http://www.sleepfoundation.org/article/ask-the-expert/pain-and-sleep
http://www.infraredsauna.net/sleep.html

About your pain relief during sleep: If your issue is allergies and sinuses there are some possibilities which you would have to explore.
It is possible that the way your head is positioned during sleep allows the sinuses to drain. It is also possible that the allergens to which your body reacts during the day are not present in your bedroom. Is it really allergies?

People with Parkinson’s disease often suffer from sleep disorders – that can often be an early symptom. These people may not get the benefit of a delta sleep because they do not reach it. If they have urinary issues, which many do – they are constantly up and down to use the bathroom – if they have rigidity or stiffness, they must struggle to get up, get out of bed, walk and the cycle continues. They are constantly tired and their pain can increase. Those who can bring the urinary issues under control often find that they can achieve a deeper sleep.

From a The Sleep Review:
"Research in genetic mitochondrial diseases shows not simply myopathic changes, but also marked hypothalamic disruption. As the hypothalamus controls sleep, the hormonal and autonomic systems, and temperature regulation, it has higher energy needs for its size than other areas. Because of this, as energy stores are depleted, hypothalamic dysfunction occurs early on, resulting in the disordered sleep, autonomic dysfunction, low body temperatures, and hormonal dysfunctions commonly seen in these syndromes. In addition, inadequate energy stores in a muscle result in muscle shortening (think of rigor mortis or writer’s cramp) and pain, which is further accentuated by the loss of deep sleep. Reductions in stages 3 and 4 of deep sleep result in secondary drops in growth hormone and tissue repair. As discussed below, disrupted sleep causes pain. Therefore, restoring adequate energy production through nutritional, hormonal, and sleep support, as well as eliminating the stresses that overuse energy (such as infections and situational stresses, etc), restore function in the hypothalamic "circuit breaker" and also allow muscles to release, thus allowing pain to resolve"
http://www.sleepreviewmag.com/issues/articles/2009-04_02.asp

We could also go into the brainwave patterns, the frequency patterns but you can read about that if you want at some sites listed and at other more technical sites.

I do think it is important that the source of your facial pain be identified.