Psychologically, sleep serves as a bridge and a safety valve between consciousness and subconsciousness. Mental tiredness is also a result of mental limitation and rigidity (too much focus can be damaging), and sleep is like a daily 'reset' for the mind and body.
What is the relationship between stress and sleep? Sleep takes care of the regular pressures of everyday life, and it can also absolve a great deal of stress related pressure.
Can sleep release stress completely? Unfortunately, that's generally not the case. The conscious mind gets some relief due to sleep, but sleep is an 'on-off' switch for everyday consciousness and not a way to consciously resolve problems.
Sometimes when stress is too overwhelming, it can partially override the mechanisms of sleep and result in a state of insomnia. There is a gap between a normal state of waking consciousness and consciousness while asleep - trying to consciously bridge that gap helps avoid mental rigidity and improves rest.
Use sleep - share the burden. Trust your own subconscious and let it handle problems within its own domain. If you do so, the sources of stress will become easier to identify and resolve.