A bulb has different watts and radiates light as per its capacity .
A man too is like a bulb he has different level of knowledge and intelligence depending on individual capacity and input.
A bulb when weak flickers.
When a man ages he is considered weak and sometimes senile.
A bulb relaxes when switched off. Likewise a man too relaxes and recuperates.
Over usage of bulb gives it a short life span.
Likewise more workload, tension, strain and...