thats an awful lot of hassle. don't give in to myth. Just cut the plant when ready and hang dry.
light... dark... doesnt matter.
There is no myth about it! The compounds on the plant that we are utilizing are comprised of the same secondary metabolites as the terpenes, terpenoids and ketones found in essential oils of other plants. They are very simple Carbon-Hydrogen-Oxygen chains meaning that they can degrade under light.
Take a look at this link, it covers it in pretty good detail the various effects on the terpenes, terpenoids, ketones and other related secondary metabolites that are harvested.
http://onlinelibrary.wiley.com/doi/10.1111/1541-4337.12006/full
"
Light
Ultraviolet (UV) light and visible (Vis) light are considered to accelerate autoxidation processes by triggering the hydrogen abstraction that results in the formation of alkyl radicals (Choe and Min
2006). Correspondingly, laurel as well as fennel oil stored in the dark (Misharina and Polshkov
2005) basically underwent the same alterations as upon storage under light, namely a decline in eugenyl acetate, estragol,
trans-anethole, and several monoterpenes as well as an augmentation of
p-cymene, eugenol, and anisaldehyde. The same was true for lemon oil in which decreasing amounts of geranial, terpinolene, and γ-terpinene together with a rise in
p-cymene have been observed (Fincke and Maurer
1974). However, compositional changes proceeded considerably faster when illumination was involved. Especially monoterpenes have been shown to degrade rapidly under the influence of light (Misharina and others
2003). The same study also reported on transformation reactions taken place in marjoram oil during storage under light that led to the formation of several unidentified minor components. Very recently, Turek and Stintzing (2011b; 2012) showed that changes in several essential oils were promoted under the impact of light, however, oils from varying plant species responded differently: While essential oil from thyme did not alter much, rosemary oil turned out to be very susceptible to imitated daylight that readily led to a changing chemical composition. This was mainly revealed by a considerable increase in
p-cymene, camphor, and caryophyllene oxide concomitant to the degradation of β-caryophyllene and the monoterpenes β-myrcene, α-terpinene, as well as α-phellandrene. Also, one minor compound in lavender oil yet not further identified totally broke down when illuminated, while another unidentified substance was initially built up during the 1st month stored under light but degraded again upon advanced storage (Turek and Stintzing
2012). Such photo-artifacts induced by sunlight or by UV irradiation with a distinct spectral distribution were identified in stored essential oils from anise (photoanethole) and lemon (photocitral) (Miething and others
1990; Nguyen and others
2009). For the former, the light-induced photoanethole, identified as 4,4’-dimethoxystilbene (Figure
5C), was suggested to result from photocycloaddition between anethole and anisaldehyde (Miething and others
1990). Moreover, in sweet fennel oil,
trans-anethole had completely oxidized to anisaldehyde or isomerized to
cis-anethole (Figure
6) after 2 mo of storage at room temperature under light (Misharina and Polshkov
2005).
trans-anethole, the main component in sweet and bitter fennel oil, was reported to be converted into
cis-anethole when treated with UV rays or high temperatures, the
cis-isomer being 10 to 12 times more toxic than the
trans-form (Braun and Franz
1999). In accordance with these data, a range of photochemically catalyzed intramolecular isomerization reactions such as cycloaddition or
trans-cis conversions of various monoterpenoids had been earlier described by Pfau (
1972)."