From Mandukya Upanishad V:
यत्र सुप्तो न कञ्चन कामं कामयते न कञ्चन स्वप्नं पश्यति तत् सुषुप्तम् । सुषुप्तस्थान एकीभूतः प्रज्ञानघन एवानन्दमयो ह्यानन्दभुक् चेतो मुखः प्राज्ञस्तृतीयः पादः ॥ ५॥
5. Where the sleeper desires not a thing of enjoyment and sees not any dream, that state is deep sleep. (The Self) seated in the state of deep sleep and called Prajna, in whom everything is unified, who is dense with consciousness, who is full of bliss, who is certainly the enjoyer of bliss, and who is the door to the knowledge (of the preceding two states), is the third quarter.
Here Shushupti is called प्रज्ञानघन. Prajnan (प्रज्ञान) means knowledge, wisdom or intelligence and Prajnanghana means nothing but knowledge. It can be interpreted as full of knowledge/consciousness or dense with (or mass of) consciousness. The self in this state is called प्राज्ञ (Prajna) which means intellectual.
But for the state of deep sleep, we can say that actually there is no* consciousness or no any state like mass of consciousness, In other words, it seems the state is full of ignorance* instead of mass of consciousness!
* Read the following verse which clearly describe shushupti as unconscious or full of ignorance.
From Chhandogya Upanishad 8.11:
तद्यत्रैतत्सुप्तः समस्तः सम्प्रसन्नः स्वप्नं न विजानात्येष आत्मेति होवाचैतदमृतमभयमेतद्ब्रह्मेति स ह शान्तहृदयः प्रवव्राज स हाप्राप्यैव देवानेतद्भयं ददर्श नाह खल्वयमेवꣳ सम्प्रत्यात्मानं जानात्ययमहमस्मीति नो एवेमानि भूतानि विनाशमेवापीतो भवति नाहमत्र भोग्यं पश्यामीति ॥ ८.११.१॥
VIII-xi-1: Prajapati said, 'He who is fully asleep, composed, serene and knows no dream, he is the Atman. He is the immortal, the fearless. He is Brahman'. Indra went away satisfied in his heart. But even before reaching the gods he saw this difficulty: 'In truth this one does not know himself now as "I am he", nor indeed these beings. It seems as if he has gone to annihilation. I see no good in this'.
स समित्पाणिः पुनरेयाय तꣳ ह प्रजापतिरुवाच मघवन्यच्छान्तहृदयः प्राव्राजीः किमिच्छन्पुनरागम इति स होवाच नाह खल्वयं भगव एवꣳ सम्प्रत्यात्मानं जानात्ययमहमस्मीति नो एवेमानि भूतानि विनाशमेवापीतो भवति नाहमत्र भोग्यं पश्यामीति ॥ ८.११.२॥
VIII-xi-2: He came back again, fuel in hand. Prajapati asked him, 'Desiring what, O Indra, have you come back, since you went away satisfied in your heart?' He replied, 'Revered sir, in truth this one does not know himself as "I am he", nor indeed these beings. It seems as if he has gone to annihilation. I see no good in this'.
So, Help me to understand why Shushupti is called प्रज्ञानघन/Prajnanaghana (mass of conscious) and the self in this state is called प्राज्ञ/Prajna (intellectual) even there is no knowledge/realization of the self and ignorance of truth.