Is there an optimal illumination level for human habitats/workplaces?
It seems there has been some work done on too much light affecting human health, but in regards to lack of darkness at night, rather than too much during the day. Without the appropriate amount of darkness at key times melatonin production and the major clock regulator or SCN can be disrupted and lose sync with the peripheral clocks. See here [1] for a comprehensive article with loads of information and other links to studies done on this topic.

This part in particular illustrates some of the problems associated with too much light:

"Overexposure to sun-mimicking light is also believed to play a role in the increase of insulin resistance, type 2 diabetes and obesity in developed countries (where round-the-clock illumination is more prevalent). Some research also shows that too much sun-mimicking light ups our craving for carbohydrates.

Before humans started cultivating grains, carbohydrate energy was available only from spring to fall. In response, our bodies evolved to stock up on carbohydrates during the growing season to prepare for winter, when no carbohydrates were available. In turn, we didn’t crave carbohydrates in the winter. All that changed with the light bulb, write T. S. Wiley and Bent Formby, PhD, in Lights Out: Sleep, Sugar and Survival (Atria, 2001)"

This might be a bit off topic but check out http://www.lightingdeluxe.com/workplace-lighting-ergonomics.html and the HSE has some info on what the required lighting conditons in the workplace should be [2]. They only mention lighting conditions with reference to reducing levels of stress in the workplace though. 

Also not directly related but pretty cool; an article in Science from 1998 found that light shining on the back of the knees may be able to regulate circadian rhythms [3].

[1] http://experiencelife.com/article/light-rhythms/

[2] (scroll to the lighting section) http://www.hse.gov.uk/humanfactors/topics/lighting.htm#lighting

[3] Wright, K.P. Jr. &amp; C.A. Czeisler. Absence of circadian phase resetting in response to bright light behind the knees. Science 297, 571 (July 26, 2002).

or if you don't have access http://www.genomenewsnetwork.org/articles/08_02/bright_knees.shtml