An occasional sinking of the consciousness happens to everybody. The causes are various, some touch from outside, something not yet changed or not sufficiently changed in the vital, especially the lower vital, some inertia or obscurity rising up from the physical parts of nature. When it comes, remain quiet, open yourself to the Mother and call back the true condition, and aspire for a clear and undisturbed discrimination showing you from within yourself the cause or the thing that needs to be set right.
Ref : Letters on Yoga Vol. III