Observations of noctilucent clouds (NLC) from northwest Europe have been collected by a network of observers for almost 40 years. Previous analyses of the observations have found an apparent increasing frequency of occurrence, a 10--11 year modulation and evidence for 5-day periodicity. Here we reexamine the observational data for NLC occurrence to test whether the observed variations can be explained by planetary wave activity in the middle atmosphere. Planetary wave amplitudes and phases in the lower mesosphere are derived from global meteorological assimilations from 1979--2000 and extrapolated to the mesopause. When the NLC observations are selected from a constant observing area, we find that there is no substantial trend in yearly NLC occurrence over the observation period, whereas the 10--11 year and 5-day modulations remain significant. We find a strong correlation between the probability of observing NLC and the combined effects of stationary, 16-day and 5-day planetary waves at the NLC location. The most reasonable explanation for the correlation is that the probability of observing NLC depends on the strength of the wind from the north, which in turn depends on the amplitude and phase of the planetary waves. The influence of planetary waves on NLC occurrence may to a certain extent explain the 10--11 year periodicity in NLC. This possibility is a consequence of a strong correlation between the phase of the stationary planetary waves and the 10--11 year cycle of solar activity during the period 1979--2000.