Visiting Machu Picchu in January, during the rainy season, we'd read to dress warm and expect rain and fog.
The day we went it turned out warm, bright and very sunny. I knew about the higher altitude putting one at risk for sunburn, so I dutifully put on my sunblock in the morning, and then smeared Aruba Aloe sunblock lip balm on my nose during the day.
Luckily I was wearing jeans and long sleeves, so my face got the worst of it. The sun is VERY strong. I wish I'd brought my hat and zinc oxide to completely block it out. I'm pretty fair, but my husband also got burned (especially on his arms where he'd pushed his sleeves up - and he'd applied sunblock there too). Some Japanese tourists were running around with pretty much just their eyes peeking out - they must have looked silly but were definitely the smart ones there.
Thankfully, the airlines LOST our sleeping bags on the flight to Lima. We were staying in Cusco for about 3 or 4 days before taking off on our trek, so we were confident our lost items would appear by the time we left for Machu Picchu. WRONG! And for this, we had no idea how lucky we were!
What happened is, we "rented" two new sleeping bags from the local travel operator with whom we were trekking the Inca Trail; they provided us with the highest quality, THERMO sleeping bags (the kind that are for 32F degree weather - I think that's 0 degrees Celsius? Sorry - I'm bad at the metric system!!). I am telling you, we would NOT have survived those cold nights in the high Andean plateaus and mountains, if we had not had these wonderfully warm sleeping bags.
So make sure your sleeping bags are top quality THERMO - it is worth the investment!
The dead woman's pass at 4200m above sea level is the highest pass on the trail. It's a good two to three hour climb from the valley floor and even with the aid of the Inca cut steps, it is a hard haul. Of course, we had the best weather of the trip this day and it was raging hot and sunny. Actually, that's not good weather for climbing...lol. I must admit I was glad I wasn't carrying the stove, tent, and food at this point.