When it's particularly cold out I often just drape my coat over the hammock RL down at the foot end to trap in warm air and it seems to help. I would imagine a PL would be a significant improvement over that. I would vent it up at the head end, ie: don't completely close your self in with it, but if you don't mind carrying it I would say it'd be worth doing.
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Deep peace of the running wave to you.
Deep peace of the flowing air to you.
Deep peace of the quiet earth to you.
Deep peace of the shining stars to you.
Deep peace without end to you.
adapted from - ancient gaelic runes