How to Sustain Alertness

I am wondering if alertness is best sustained in terms of: 1. Attempting to sustain alertness in meditation, not meditating when too lethargic. In other words, always exerting oneself and keeping sharpness when in meditation, and stopping when the mind becomes unclear. 2. Or, engaging with meditation even when lethargic, continuing one's meditation despite the mind being unclear and hazy. In other words, waiting for the mind to clear up. Which is more accurate?