We generally know Parkinson's as a motor-symptom disease. But a significant part of the disease has non-motor symptoms that we should address as well.
One of the common disorders of Parkinson's involves sleep...or lack thereof. And sleep disorders of PD can start well before diagnosis or motor symptoms appear, sometimes even years.
Sleep disorders can include, but are not limited to, dream-acting-out (REM behavior), sleep apnea, difficulty falling/staying asleep, excessive daytime sleepiness and fatigue in general. Because PD affects the muscles and movement of every part of the body, sometimes the eyelids will become too heavy to keep open. This is a condition called Blepharospasm, where uncontrollable spasms force the eyelids to close. This sometimes accompanies PD.
Some of the undesirable sleepiness is due to the fact that your loved one isn't getting good sleep at night. For instance, even though dream-acting-out is a behavior that only occurs during the deepest sleep, the talking and acting out during REM actually causes more fatigue and can actually cancel out the deep sleep your loved one should be getting.
Changing medications; stopping some, and even changing the time the meds are taken, can make a difference. For example, if you take vitamin D at night, you might start taking it at the beginning of the day instead, as vitamin D is like a shot of sunlight and promotes wakefulness. Your neurologist or GP should be able to help you make wise choices about what might work better for you.
Some other suggestions might include increasing socialization, exercise, mentally stimulating activities, or lifestyle modification. Whatever you and your doctor work out will be the best way to handle the daytime sleepiness for you.
Just remember, exercise is always the best solution for every Parkinson's symptom!