Time period of a hammock

 

I was laid up in this very comfortable hammock. I timed 10 complete oscillations at 18 seconds. Can this be modelled as a pendulum? Using the formula T=2.pi Root(L/g) would give L=80cm. Modelling for a point mass in the middle I'd estimate that the length should be twice that. One thought is whether it is the length of the pendulum string that matters or the vertical height below the suspension point. If it is the latter, then the estimate will be about right. I need to do some maths!