A sneeze, is a semi-autonomous, convulsive expulsion of air from the lungs through nose & mouth, usually caused by foreign particles (dust, pollen/other tiny particles in air) irritating the nasal mucosa; often happens suddenly & without warning. Sneezing is linked to sudden exposure to bright light, sudden change (fall) in temperature, breeze of cold air, a particularly full stomach, or viral infection, & can lead to the spread of disease. 

Sneezing typically occurs when foreign particles or sufficient external stimulants pass through the nasal hairs to reach the nasal mucosa, triggers the release of histamines, which irritate the nerve cells in the nose, resulting in signals being sent to the brain to initiate the sneeze, via Trigeminal nerve. Biological clocks - dictated by circadian rhythms - move in concert with the immune system, prompting timely sneezing bouts. Every time we go to sleep, our immune systems get a reset. Like the rest of our bodies, the system down shifts into a lower gear during this period during which we are fighting fewer outside invaders. Endogenous circadian rhythm in the immune system, generally tends to rev up in the later stages of sleep, but we do not notice until we wake.

It looks like body has a sense of when it is about to wake up & greet the day-& all of the allergens that come with it. People who suffer from bouts of sneezing at exactly the same time every day, have problem in morning. Besides allergic & other rhinitis, Flu & viral inf, sinusitis, deviated septum, polyps, bleeding, pollution, chemicals, smoke etc r common causes.