Final answer:
Rukmani is unable to sleep at the temple due to the feeling of fear, a common disruptor of sleep and one of the seven universal emotions.
Step-by-step explanation:
The feeling or experience that keeps Rukmani from being able to sleep at the temple is fear. This emotional response is not only common in literature but also in real-life situations, where the presence of fear can disrupt an individual's sleep. Fear is also known as one of the seven universal emotions that people can experience, though it doesn't seem to be explicitly mentioned in the provided references, it can be inferred from the context given. For instance, in the provided text, the character's inability to sleep due to 'an incubus of utterly causeless alarm' closely parallels the feeling of fear that can stem from various psychological factors, be it due to the subconscious or a real, identifiable threat.