Updating stamina in UISlider

Hi all,

I'm fairly new to Unity (and coding in general), so please bare with me.

I've created a very basic environment for an RPG, to which I am adding functionality as a way of learning new concepts.

At present, I'm trying to add a UISlider to reflect the characters stamina. The stamina needs to fall as the player holds left shift, and regenerate when they release the key. I have set up the canvas, and positioned the slider. I have created a 'PlayerStats' script to hold the logic, and attached it to the player.

My problem is that the bar does not update when I use time.deltaTime - but does when I use time.time - which doesn't seem appropriate. Also, changing the float values of staminaFallRate and staminaIncreaseRate has no effect on the speed of the decrease/increase?

It would be really great if someone could please be so kind as to lend their thoughts :)

Since I am a true beginner, I ask you to also point out any other shortfalls/floors/inefficiencies in my syntax! Thank you!

 public class PlayerStats : MonoBehaviour {
 
     public int startingHealth = 100;
     public int currentHealth;
 
     public float startingStamina = 100f;
     public float currentStamina;
     public float staminaFallRate = 150f;
     public float staminaIncreaseRate = 2f;
 
     public int startingArcane = 100;
     public int currentArcane;
 
     public Slider healthBar;
     public Slider staminaBar;
     public Slider manaBar;
     public Image playerHurtImage;
     public AudioClip deathClip;
     public float flashSpeed = 5f;
     public Color flashColor = new Color (1f, 0f, 0f, 0.1f);
 
     AudioSource playerAudio;
     ThirdPersonCharacter thirdPersonCharacter; // Used to stop player from running around once they are dead.
     bool isDead;
     bool damaged;
     bool isRunning;
 
     void Awake()
     {
         thirdPersonCharacter = GetComponent<ThirdPersonCharacter>(); // Third person character is name of script we are getting.
         playerAudio = GetComponent<AudioSource>();
         currentHealth = startingHealth;
         currentStamina = startingStamina;
         staminaBar.value = currentStamina;
         currentArcane = startingArcane;
 
     }
 
     void Update()
     {
 
         if(Input.GetKey(KeyCode.LeftShift))
         {
             currentStamina -= Time.time / staminaFallRate;
             staminaBar.value = currentStamina;
         } 
         else if (currentStamina < startingStamina)
         {
             //staminaBar.value = Mathf.MoveTowards(currentStamina, startingStamina, 2f);
             currentStamina += Time.time / staminaIncreaseRate;
             staminaBar.value = currentStamina;
         }
 
         if (currentStamina >= startingStamina)
         {
             currentStamina = 100;
         }
     }