Why not just use something similar to the link you posted then.
Mean= 600
SD= 100
lowest value =0
max = infinite
99.7-6 of values would lie between 300-900 with a range of 0-Inf.
Adding a max value would also be useful because you wouldn't want a sleep of 100000, even if it's statically unlikely it will eventually occur.
I think the method is useful if used and built correctly with a Min and a Max value
for example:
Mean- 600
SD- 200
Min- 0
Max- 5000