Lighting:

The small, head-mounted device measures an individual's daily rest and activity patterns, as well as exposure to short-wavelength light that stimulates the circadian system. The device measures activity and light together at regular time intervals and electronically stores and logs its operating temperature. The Daysimeter can gather data for up to 30 days for analysis.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". How many days can the Daysimeter gather for analysis?
30