Why am I getting sunburns more easily?

I didn't actually get a sunburn, but I am questioning a prevalent trend of my body.

I used to get sunburns a lot less easily - doing outdoor activities in the HK/California sun without sunscreen was never a problem. I did get a little darker, but it was nothing. There was not the slightest redness that occurred afterwards.To keep a fair comparison, I did not use significant amounts of sunscreen anytime.

I have come up with several explanations -

• Overall Skin aging. Everybody's skin ages. Big UV exposure in the past increases aging in the future.
• Lack of extra secretions - from 10 years old onwards,  I experienced an increasing amount of body oil secretions, and sweat. This protects the skin in general and is the body's natural protection against the sun. After 19years old, I have reduced amounts of skin oil, so that is less protection.
• Repeated sun exposure - I did not have any winter this year. Winter is a great way to let everything go to normal. Going to Singapore, a tropical area where there is sun almost everyday means lots of repeated exposures. Coming back to the HK summer, there wasn't time to recover fully. I also hike on a daily/semi-daily basis, so that gives me about 1 hour of UV exposure daily, which is quite a lot on most days. I wear a hat and use spray on sunscreen, but wiping sweat may wipe it off too. Diving in the summer also makes sun protection difficult, because any sunscreen going to get rubbed off and dissolved away in the sea.
• Dryer skin - I find that I do have somewhat dry skin. Dry skin is more susceptible to damage. 
• UV hole in the sky? - Maybe there is more UV coming through?
• Genetic reasons - Some people get sunburnt more easily than others.
• Diet? Does the changes in food production (GMO, artificial hormones, antibiotics, artificial food additives) lead to more skin problems/photosensitivity of skin?
• Sleeping late? Sleeping later than 0000 reduces the time there is to repair skin and related organs. Also more light exposure from light sources. This leads to less strong skin.

To conclude, I find the Repeated Sun Exposure, diet, sleeping late, decrease in skin secretions, dryer skin and overall skin aging to be the reasons why I am getting sunburns more easily, in order of importance.

The easiest fixes would be to sleep a little earlier, eat better, use a moisturizer and apply sunburn measures more frequently while doing outside sports.