Lisbon to Valencia is +- 900km by road as the fly crows.
There are no direct train overnight trains.
One would have to take the overnight train from Lisbon and then a connecting train to Valencia.
I have checked the renfe.es timetables and all routes would require arriving at Madrid Charmartin Station, taking the Metro to Puerto de Atocha station and then catching another train to Valencia. Transit time would be about 20 hours.
TAP-Air Portugal (www.flytap.com) flies direct daily(twice daily on weekdays) from Lisbon to Valencia.
Flight time is 90 minutes.
Alternatively, one would have to route via Madrid Barajas Airport. Iberia.com, together with its subsidiary, Air Nostrum offer one-stop connections with transit time starting at +-5hrs.