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We report the first lidar observations of regular occurrence of mid-latitude thermosphereionosphere Na (TINa) layers over Boulder (40.13°N, 105.24°W), Colorado. Detection of tenuous Na layers (~0.1-1 cm-3 from 150 to 130 km) was enabled by high-sensitivity Na Doppler lidar. TINa layers occur regularly in various months and years, descending from ~125 km after dusk and from ~150 km before dawn. The downward-progression phase speeds are ~3 m/s above 120 km and ~1 m/s below 115 km, consistent with semidiurnal tidal phase speeds. One or more layers sometimes occur across local midnight. Elevated volume mixing ratios above the turning point (~105-110 km) of Na density slope suggest in situ production of the dawn/dusk layers via neutralization of converged Na+ layers. Vertical drift velocity of TINa+ calculated with the Ionospheric Connection Explorer Hough Mode Extension tidal winds shows convergent ion flow phases aligned well with TINa, supporting this formation hypothesis. Plain Language Summary Thermosphere-ionosphere metal (TIMt) layers are an intriguing phenomenon discovered 10 years ago in Antarctica. They provide a natural laboratory and tracer to study the complex space-atmosphere interactions in the Earth's thermosphere and ionosphere. TIMt layers have been observed in the neutral Fe (iron), Na (sodium), and K (potassium) species in both the polar and tropical regions but quite rarely at midlatitudes. To date, all reported TIMt layers have an irregular occurrence. This article reports an exciting new discovery made at a mid-latitude site. That is, the TIMt layers in the Na species (thermosphere-ionosphere Na layers) occur fairly regularly before dawn and after dusk. Such regular occurrence of thermosphere-ionosphere Na (TINa) layers is reported for the first time. Such layers are tenuous and their detection is enabled by a Na Doppler lidar developed at the University of Colorado Boulder, which possesses very high detection sensitivity. Utilizing the thermospheric wind data from a new satellite mission (Ionospheric Connection Explorer), it is found that these neutral metal layers are closely related to the tides-driven metal ion layers. The Boulder TINa layers likely provide a new way to study the ion transport in the E to lower F regions. [ABSTRACT FROM AUTHOR]