As a USA citizen, I am not allowed to enter Portugal (due to Covid-19 risk, regardless of my apparent health).
But I have a 25 hour layover in Lisbon, so I am thinking to search the international transfers area for a food court bench on which to sleep. I doubt that I will get more than an hour of sleep, so lack of sleep will probably make me sick.
Are there any better sleeping options?
You're not getting on the plane to Lisbon!
Per TIMATIC, the database used by airlines, transit in Portugal is possible for:
Passengers arriving from a non-Schengen Member State with a confirmed onward ticket for a flight within 24 hours to a third country which is not a Schengen Member State
Furthermore, you're not exempt from the entry ban (you would be in the other direction, however, i.e. arriving from the UK)
So contact TAP ASAP and modify the booking.
As for facilities at Lisbon airport, there's a food court inside security as well as couches near gate 16 and a rest zone near gates 19-20. I can't seem to find which gates are in the non-Schengen sector, however.
The other answer is incorrect. You will not be prevented from boarding the airplane. The TWOV restriction it cites does not apply to you because you qualify for a visa exemption:
Nationals of Albania, Andorra, Argentina, Australia, Bosnia and Herzegovina, Brunei Darussalam, Canada, Chile, Colombia, Costa Rica, Dominica, El Salvador, Grenada, Guatemala, Honduras, Israel, Japan, Kiribati, Korea (Rep.), Malaysia, Marshall Isl., Mexico, Micronesia (Federated States), Monaco, Montenegro, New Zealand, Nicaragua, North Macedonia (Rep.), Palau, Panama, Paraguay, Peru, Samoa, San Marino, Singapore, Solomon Isl., St. Lucia, St. Vincent and the Grenadines, Timor-Leste, Tonga, Trinidad and Tobago, Tuvalu, USA, United Arab Emirates, Uruguay, Vanuatu, Vatican City (Holy See) and Venezuela for a maximum stay of 90 days. (SEE NOTE 51200).
NOTE 51200: The maximum stay is granted within 180 days.
Your best bet is to follow the suggestion by Arthur's Pass in a comment:
You could take a look at Sleeping in Airports. They warn that the exact situation can change at short notice. – Arthur's Pass Aug 23 at 15:40
