We can only agree with the comments by Erren et al., that causal conclusions cannot be drawn on the basis of cross-sectional data. Overall, we therefore attempted throughout our entire manuscript to avoid any implications of causality. In order to identify causal associations—especially in such a complex subject—results from longitudinal studies are required. Furthermore, a detailed assessment of sleep characteristics is desirable, and this is feasible to a limited extent only when using a questionnaire. The question also remains unanswered about the time period for which sleep changes would have to prevail in order to prove associations with muscle mass, especially in older persons.
Still, cross-sectional analyses are not only legitimate but have important functions in medical research. The associations found here, which are supported by pre-studies and theoretical explanatory models, do not permit causal conclusions, but they enable the gaining of insights that will then have to be proved or refuted in longitudinal studies. In the context of the Berlin Aging Study II (BASE-II), all that is currently available is cross-sectional information; the planned longitudinal observational study may provide deeper insights into the underlying mechanisms. Overall we agree with your comments and support your call for longitudinal studies of this complex topic in order to figure out causalities.
Footnotes
Conflict of interest statement
The authors declare that no conflict of interest exists.
References
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