The sequence of numbers produced by randn is determined by the internal state of the uniform pseudorandom number generator that underlies rand, randi, and randn. randn uses one or more uniform values from that default stream to generate each normal value. Control the default stream using its properties and methods. See @RandStream for details about the default stream.
Resetting the default stream to the same fixed state allows computations to be repeated. Setting the stream to different states leads to unique computations, however, it does not improve any statistical properties. Since the random number generator is initialized to the same state every time MATLAB software starts up, rand, randn, and randi will generate the same sequence of numbers in each session until the state is changed.
On that page there is an example of how to change the default stream of numbers using clock: