text: The ATPase PINA is expressed at high levels under this stimulus.
full_question: The ATPase PINA is expressed at high levels under this stimulus. The DLMO response to this stimulus triggersPRC phase-advance. Some organisms occupy the epipelagic zone during diel vertical migration in responseto this exogenous stimulus. Phacelia is dependent on this stimulus for germination, and pitaya flowersonly bloom when it is present. A condition appearing under this stimulus is often the first symptom of retinitispigmentosa. A steady influx of sodium through certain cGMP-gated channels occurs under its namesake"current" and is reversed by activation of transducin by opsin. This phenomenon increases pineal gland secretionof melatonin, which stimulates sleep. For 10 points, name this phenomenon during which nocturnal animalsare active. 
first_sentence: The ATPase PINA is expressed at high levels under this stimulus.
raw_answer: darkness [or synonyms of darkness, such as absence of light and scotopic conditions; or nighttime; or twilight; or day turning into night; or light turning into darkness; prompt on nocturnality, circadian rhythms, and sleep-wake cycle until "exogenous" is read; prompt on light/dark cycle]