That may seem like a warm temp to be sleeping cool in, but it can happen. It does depend on the type of sleeper you are [warm or cold] and how much exposure to wind you have. Some sleep warmer than others and a well placed tarp or careful site selection can increase your comfort. But when temps start hovering around that 60-70F mark, you've got to be prepared for adding something to your hammock sleep system or you can end up too cold for comfort. And that's not what anyone wants.
I was out for an overnight back in early March with overnight lows at 32F. I was completely unprepared and had nothing other than my 5F degree sleeping bag and a blow up mattress to keep me warm. It was one of the coldest nights I'd ever spent outside and though it was miserable, I did survive and learned a ton of lessons.
Now, I was just out for a night this past weekend and the overnight low was 73F. For me, that temp was perfect as I stayed warm and comfortable the whole night and even had part of my 5 degree sleeping bag left open. Those nights are disappearing quickly now in Upstate New York so it's time to add a few pieces to my sleep system to keep me out on the trail all season long.
The simple, yet somewhat pricey answer to staying warm when temps start dropping is the down or synthetic down under quilt [pictured above]. You can look to spend between $100-$250 on a 3 season under quilt which can take you comfortably down to 20 to 40F but I've got a few low-cost ideas to extend your season into the 50F range without spending more than $25.
I've got to first tell you that I'm a cool season camper. I'd much rather be out when it's below 50F and even during mid winter it's more fun for me. I've had some hard times learning how to stay warm in my hammock over this past season but I found a few ways that are super effective and very low cost.
Nalgenes are great to bring along once the weather really starts dipping down because they are super robust and will resist splitting even if you leave water in them and leave them out in the cold.
But a reasonably well-known trick for staying warm is to first heat up 2-3 cups of water to boiling then pouring it into your Nalgene before retiring for the night. The wool sock is great at insulating you against burns and in prolonging the time that the water stays warm.
When it's time to cuddle up with your Nalgene for the night, stuff it in that wool sock and cradle the bottle in your crotch. It may sound a little strange but running down the inside of both your thighs is the femoral artery which is a major transport artery for blood to the body. By placing the Nalgene there you're actually warming the blood as it circulates through your body.
Warming yourself up this way is especially effective when you're already cold as that warmed blood reaches out to all your extremities, especially those cold feet. And you can expect the Nalgene water bottle to keep you warm for up to 8 hours.
Chances are you already own a Nalgene and a wool sock so you haven't even spent any money yet. Note that the bottle has to be a Nalgene - others will melt and deform if you place boiling water in them.
No matter how much down you've got in your sleeping bag or what it's rated for, when sleeping in a hammock that insulation under you gets compressed and renders it near ineffective.
Down's loft is one of the qualities that makes it so effective at keeping the body warm and you just can't keep it lofty when it's between you and the hammock. The Yoga style mat, however, with it's closed cell foam, excels at staying lofted even when compressed. Most yoga mats are either 1/4",3/8" or 1/2" - the thicker you can find, the better.
Slip the Yoga Mat under your sleeping bag before you get in and that extra 1/2" of closed cell foam can make it feel 10 degrees warmer. Add the Nalgene bottle and you'll be good down to 60F.
Reflextix is a super low weight, low cost insulation that is carried in most home improvement stores in 24" x 10'-0" rolls for under $20. Normally used for ductwork insulation, there are many hammock campers, including me, that swear by it's heat reflecting properties.
To make it the most effective, cut a piece to length according to your height, tapering the head end and foot end similar to a mummy bag shape. On the sides from shoulder to elbow, add a second piece from the leftover scraps about 6" to 8" wide and duct tape it in place. When in you hammock, those 'wings' will cradle the backs of your arms and shoulders to keep them warm.
There are some members of Hammock Forums that attest to Reflectix keeping them warm even when night temps dip into the 30s. A camping buddy of mine made me my reflectix pad after I told him about that super cold night in March. It's been an amazingly effective, low cost solution to keeping me on the trail even when the nights get cold.
If you've got a double layer hammock, the yoga mat and reflectix are best placed in between the hammock layers. When it's tucked away in there it won't shift around and even if you move around at night, both will stay comfortably beneath you. If you're using both, keep the reflectix above the yoga mat so that it can best reflect your own body heat.
Any one of these measures can add 5-10 degrees to your hammocking season. All three could add an entire season and you won't spend more than $25.