ashleyk writes:

"It is my understanding that regular sinemet has a half life of 1 hr ..."

That's about right for the plasma half-life. But, fortunately, the clinical effect of the drug lasts longer: perhaps, 4 hours in the early days, but less as the disease progresses. Brooks [1] writes:

"In the early stages of PD, this short plasma half-life is discordant with the clinical effect of the drug, which may persist for many hours, indicating that surviving dopaminergic neurons are able to store the exogenous dopamine generated from levodopa and so buffer variations in levodopa availability. However, with progression and further neuronal degeneration, the levodopa-buffering capacity is lost and deep troughs in plasma levodopa levels correspond with deep troughs in dopamine within the striatum, resulting in pulsatile stimulation of striatal dopaminergic receptors".

A combination of daily rasagiline and controlled release ropinirole, and Stalevo taken four times a day works for me, 9 years post DX.

Reference

[1] Neuropsychiatr Dis Treat. Feb 2008; 4(1): 39–47.
"Optimizing levodopa therapy for Parkinson’s disease with levodopa/carbidopa/entacapone: implications from a clinical and patient perspective"
David J Brooks
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2515910/

John