**Core Concept**
Normal diurnal variation in intraocular pressure (IOP) refers to the fluctuation in IOP throughout the day. This variation is influenced by the balance between aqueous humor production and outflow, as well as other physiological factors.

**Why the Correct Answer is Right**
The correct answer is **C. Lower in the morning and higher in the evening**. This is because the IOP follows a circadian rhythm, peaking in the late afternoon to early evening and decreasing during sleep. The decrease in IOP during sleep is thought to be due to the reduction in aqueous humor production and the increase in outflow, which is facilitated by the increased pressure in the episcleral veins during sleep. Additionally, the ciliary body, which produces aqueous humor, is less active during sleep, contributing to the decrease in IOP.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because IOP does not typically decrease during the day and increase at night. While IOP can fluctuate throughout the day, the overall trend is for IOP to be higher in the late afternoon to early evening.

**Option B:** This option is incorrect because IOP does not typically remain constant throughout the day. While IOP can be relatively stable in some individuals, the normal diurnal variation in IOP is characterized by fluctuations throughout the day.

**Clinical Pearl / High-Yield Fact**
It is essential to consider diurnal variation in IOP when interpreting glaucoma patients' measurements, as the IOP can be influenced by the time of day.

**Correct Answer: C. Lower in the morning and higher in the evening**